workflows: add cancel-pr-tests to cancel CI on PR closure

[ZhongRuoyu]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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Continue running the tests after a PR is closed is a waste of our CI resources, so let's cancel the tests when a PR is closed.

The check-ci-status command has been refactored to support this.

To try the updated check-ci-status command:

export CI=1 GITHUB_REPOSITORY=Homebrew/homebrew-core GITHUB_OUTPUT=/dev/null
# Existing functionality: check if long-timeout label can be removed
brew check-ci-status --debug --long-timeout-label $pr_number
# New: check if workflow on CI is cancellable
# If true, return its workflow ID; otherwise, return null
brew check-ci-status --debug --cancel $pr_number

[MikeMcQuaid]

Looks good to me, thanks!

[ZhongRuoyu]

Let's merge this; I'll try it out in a test PR.

[ZhongRuoyu]

This workflow works 🎉 (see https://github.com/Homebrew/homebrew-core/actions/runs/6371658945?pr=148503).

However, it doesn't take care of the case where the PR is closed and later reopened, because the current logic in the check-ci-status command always checks the first run (which in other cases should be the only one). See #148503 (comment) for details. #148530 should fix this. (Update: yes, this is now fixed.)