{{branch}} is not set on pull request events

### Troubleshooting

Before submitting a bug report please read the [Troubleshooting doc](https://github.com/docker/build-push-action/blob/master/TROUBLESHOOTING.md).

### Behaviour

Using `{{branch}}` as outlined in https://github.com/docker/metadata-action#global-expressions does not work when pushing commits to a branch with a PR open.

#### Steps to reproduce this issue

1. Open a PR for new branch
2. Push commits
3. Observe output of workflow

#### Expected behaviour

Docker tags should be outputted as follows assuming a branch `foo` and sha `12345`

```
my/repo:12345-amd64
my/repo:foo-amd64
my/repo:foo-12345-amd64
```

#### Actual behaviour

`{{branch}}` is empty so the tags are as follows:

```
my/repo:12345-amd64
my/repo:-amd64
my/repo:-12345-amd64
```

### Configuration

```yml
name: docker-metadata-test
on:
  push:
    branches: ["main"]
  pull_request:
    branches: ["*"]
jobs:
  name: docker-metadata-test
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v3
    - name: Docker meta
       id: meta
       uses: docker/metadata-action@v3
       with:
         images: my/repo
         tags: |
           type=raw,value={{sha}}-amd64
           type=raw,value={{branch}}-amd64
           type=raw,value={{branch}}-{{sha}}-amd64
```

### Logs

<details>
<summary>metadata step of logs with some parts redacted</summary>

```
2022-04-20T00:00:44.3789494Z ##[group]Run docker/metadata-action@v3
2022-04-20T00:00:44.3789795Z with:
2022-04-20T00:00:44.3790108Z   images: my/repo
2022-04-20T00:00:44.3790585Z   tags: type=raw,value={{sha}}-amd64
type=raw,value={{branch}}-amd64
type=raw,value={{branch}}-{{sha}}-amd64

2022-04-20T00:00:44.3791171Z   github-token: ***
2022-04-20T00:00:44.3791392Z ##[endgroup]
2022-04-20T00:00:44.8175981Z ##[group]Context info
2022-04-20T00:00:44.8232603Z eventName: pull_request
2022-04-20T00:00:44.8232917Z sha: 137b7b03cf19010d9f14a6315a7d8d7741b99b4a
2022-04-20T00:00:44.8233160Z ref: refs/pull/80/merge
2022-04-20T00:00:44.8233385Z workflow: docker-metadata-test
2022-04-20T00:00:44.8233598Z action: meta
2022-04-20T00:00:44.8233787Z actor: evandam
2022-04-20T00:00:44.8233989Z runNumber: 124
2022-04-20T00:00:44.8234191Z runId: 2192825535
2022-04-20T00:00:44.8234662Z ##[endgroup]
2022-04-20T00:00:44.8235061Z ##[group]Processing tags input
2022-04-20T00:00:44.8235683Z type=raw,value={{sha}}-amd64,enable=true,priority=200
2022-04-20T00:00:44.8236093Z type=raw,value={{branch}}-amd64,enable=true,priority=200
2022-04-20T00:00:44.8236831Z type=raw,value={{branch}}-{{sha}}-amd64,enable=true,priority=200
2022-04-20T00:00:44.8237290Z ##[endgroup]
2022-04-20T00:00:44.8237665Z ##[group]Processing flavor input
2022-04-20T00:00:44.8237895Z latest=auto
2022-04-20T00:00:44.8238091Z prefix=
2022-04-20T00:00:44.8238279Z prefixLatest=false
2022-04-20T00:00:44.8238484Z suffix=
2022-04-20T00:00:44.8238678Z suffixLatest=false
2022-04-20T00:00:44.8239104Z ##[endgroup]
2022-04-20T00:00:44.8326167Z ##[group]Docker image version
2022-04-20T00:00:44.8326749Z 137b7b0-amd64
2022-04-20T00:00:44.8327230Z ##[endgroup]
2022-04-20T00:00:44.8348648Z ##[group]Docker tags
2022-04-20T00:00:44.8349280Z my/repo:137b7b0-amd64
2022-04-20T00:00:44.8349838Z my/repo:-amd64
2022-04-20T00:00:44.8350355Z my/repo:-137b7b0-amd64
2022-04-20T00:00:44.8351069Z ##[endgroup]
2022-04-20T00:00:44.8353443Z ##[group]Docker labels
2022-04-20T00:00:44.8353828Z org.opencontainers.image.title=my_repo
2022-04-20T00:00:44.8354284Z org.opencontainers.image.description=description
2022-04-20T00:00:44.8354824Z org.opencontainers.image.url=https://github.com/my/repo
2022-04-20T00:00:44.8355332Z org.opencontainers.image.source=https://github.com/my/repo
2022-04-20T00:00:44.8355840Z org.opencontainers.image.version=137b7b0-amd64
2022-04-20T00:00:44.8356377Z org.opencontainers.image.created=2022-04-20T00:00:44.822Z
2022-04-20T00:00:44.8356850Z org.opencontainers.image.revision=137b7b03cf19010d9f14a6315a7d8d7741b99b4a
2022-04-20T00:00:44.8357230Z org.opencontainers.image.licenses=
2022-04-20T00:00:44.8357770Z ##[endgroup]
2022-04-20T00:00:44.8358909Z ##[group]JSON output
2022-04-20T00:00:44.8359141Z {
2022-04-20T00:00:44.8359395Z   "tags": [
2022-04-20T00:00:44.8359898Z     "my/repo:137b7b0-amd64",
2022-04-20T00:00:44.8360652Z     "my/repo:-amd64",
2022-04-20T00:00:44.8361195Z     "my/repo:-137b7b0-amd64"
2022-04-20T00:00:44.8361544Z   ],
2022-04-20T00:00:44.8361757Z   "labels": {
2022-04-20T00:00:44.8363964Z     "org.opencontainers.image.title": "my_repo",
2022-04-20T00:00:44.8364504Z     "org.opencontainers.image.description": "description",
2022-04-20T00:00:44.8364981Z     "org.opencontainers.image.url": "https://github.com/my/repo",
2022-04-20T00:00:44.8365506Z     "org.opencontainers.image.source": "https://github.com/my/repo",
2022-04-20T00:00:44.8366099Z     "org.opencontainers.image.version": "137b7b0-amd64",
2022-04-20T00:00:44.8366684Z     "org.opencontainers.image.created": "2022-04-20T00:00:44.822Z",
2022-04-20T00:00:44.8367285Z     "org.opencontainers.image.revision": "137b7b03cf19010d9f14a6315a7d8d7741b99b4a",
2022-04-20T00:00:44.8367724Z     "org.opencontainers.image.licenses": ""
2022-04-20T00:00:44.8368040Z   }
2022-04-20T00:00:44.8368250Z }
2022-04-20T00:00:44.8368809Z ##[endgroup]
2022-04-20T00:00:44.8370169Z ##[group]Bake definition file
2022-04-20T00:00:44.8370447Z {
2022-04-20T00:00:44.8371202Z   "target": {
2022-04-20T00:00:44.8371777Z     "docker-metadata-action": {
2022-04-20T00:00:44.8372031Z       "tags": [
2022-04-20T00:00:44.8372504Z         "my/repo:137b7b0-amd64",
2022-04-20T00:00:44.8373187Z         "my/repo:-amd64",
2022-04-20T00:00:44.8373693Z         "my/repo:-137b7b0-amd64"
2022-04-20T00:00:44.8374050Z       ],
2022-04-20T00:00:44.8374327Z       "labels": {
2022-04-20T00:00:44.8374689Z         "org.opencontainers.image.title": "my_repo",
2022-04-20T00:00:44.8375124Z         "org.opencontainers.image.description": "description",
2022-04-20T00:00:44.8375657Z         "org.opencontainers.image.url": "https://github.com/my/repo",
2022-04-20T00:00:44.8376194Z         "org.opencontainers.image.source": "https://github.com/my/repo",
2022-04-20T00:00:44.8376702Z         "org.opencontainers.image.version": "137b7b0-amd64",
2022-04-20T00:00:44.8377253Z         "org.opencontainers.image.created": "2022-04-20T00:00:44.822Z",
2022-04-20T00:00:44.8377729Z         "org.opencontainers.image.revision": "137b7b03cf19010d9f14a6315a7d8d7741b99b4a",
2022-04-20T00:00:44.8378187Z         "org.opencontainers.image.licenses": ""
2022-04-20T00:00:44.8378466Z       },
2022-04-20T00:00:44.8378768Z       "args": {
2022-04-20T00:00:44.8379288Z         "DOCKER_META_IMAGES": "my/repo",
2022-04-20T00:00:44.8379716Z         "DOCKER_META_VERSION": "137b7b0-amd64"
2022-04-20T00:00:44.8380053Z       }
2022-04-20T00:00:44.8380309Z     }
2022-04-20T00:00:44.8380513Z   }
2022-04-20T00:00:44.8380792Z }
2022-04-20T00:00:44.8381452Z ##[endgroup]
```

</details>

> Download the [log file of your build](https://docs.github.com/en/actions/managing-workflow-runs/using-workflow-run-logs#downloading-logs) and [attach it](https://docs.github.com/en/github/managing-your-work-on-github/file-attachments-on-issues-and-pull-requests) to this issue.


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{{branch}} is not set on pull request events #191

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{{branch}} is not set on pull request events #191

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