576828e ci: test-each-commit merge base optional (Sjors Provoost)
e9bfbb5 ci: forks can opt-out of CI branch push (Cirrus only) (Sjors Provoost)

Pull request description:

  Maintainer note: `SKIP_BRANCH_PUSH=true` must be set in Cirrus for `bitcoin-core/gui` before merging this. See `https://cirrus-ci.com/github/bitcoin-core/gui` -> Settings.

  ---

  I find myself making pull requests against my fork (mostly on top of #28983, or asking others to do so. Currently only the Github actions are run on forks, because we use self-hosted runners for the Cirrus tasks.

  While setting up my own self-hosted runners for my fork, I ran into a number of issues. Some of those were addressed by #29441, but remaining issues are:

  1. When PRs are opened in the fork, cirrus CI jobs are run twice because PRs and branches reside in the same repository, rather than a main repository and a fork repository, as is the case with bitcoin/bitcoin PRs. Fix this by adding a `SKIP_BRANCH_PUSH` configuration option that allows skipping CI runs not directly associated with a PR. The fix is a generalization of [#20328](#20328), which fixed a similar problem for the bitcoin-core/gui mirror repository, and it allows removing a hardcoded reference to that repository.

      Github actions jobs will still run twice despite this change, see [#29274 (comment)](#29274 (comment)). Initially this PR tried to prevent that with b9fdd0d, but this had some potentially negative side effects, see [#29274 (comment)](#29274 (comment)), so that commit was dropped for now.

  2. When PRs are opened in the fork, the "test-each-commit" github action can fail due to not being able to find a recent merge commit. This problem doesn't happen in the bitcoin/bitcoin repository because branches in this repository used as the base for pull requests always point at merge commits.

  This PR replaces #29259 using the self hosted workers via Cirrus instead of Github.

  You can see this PR in action on this pull request to my fork: Sjors#30

  To test it yourself:

  1. spin up at least two [self hosted runners](https://github.com/cirruslabs/cirrus-cli/blob/master/PERSISTENT-WORKERS.md). Either use a seperate VM for each, or give them their own user.
  3. Install Podman and other CI dependencies (see .cirrus.yml)
  4. Give Cirrus access to your fork at https://cirrus-ci.com/settings/github/YOU
  5. Get a token from Cirrus and use it to start your worker(s)
  6. Optionally set SKIP_BRANCH_PUSH=true ~and NO_ARM=true~ env variables (see .cirrus.yml)
  make a pull request to your own fork, with this PR as the base branch

  Security wise: when dealing with code from strangers on the internet, review it first before running the CI. There's a Cirrus check-box that requires approval for people without write access to trigger CI.

ACKs for top commit:
  maflcko:
    ACK 576828e
  ryanofsky:
    Code review ACK 576828e.

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