Describe the bug
GitHub releases a new environment variable: GITHUB_REF_NAME.
Since this action also uses GITHUB_REF_NAME name.
The GitHub version cannot be overwritten by this action, and the export is silently discarded.
The action version doesn't fill this variable the same way GitHub does.
And the current content doesn't match the content produced by this action (before the GitHub release) on pull request event.

On GitHub-side, GITHUB_REF_NAME is GITHUB_REF without the refs prefix.

• pull-request event GITHUB_REF = refs/pull/42/merge
• GitHub version of GITHUB_REF_NAME = 42/merge
• Action version of GITHUB_REF_NAME = <Name of the head branch of PR 42>

To Reproduce
Use this action on a pull request-based workflow.

Expected behavior
GITHUB_REF_NAME contains the branch behind the Pull request.