Update cleanup after PR closed

[aaemnnosttv]

Summary

Follow up to PR #1172

This PR makes a few small changes to fix the GitHub Action workflow that runs when a PR is closed.

• github.head_ref is used instead of github.ref as this will always refer to the ref of the PR branch where github.ref will refer to the current branch (develop, master, etc). We should continue to use github.ref on the build-pr workflow as this runs on select branch pushes as well.
  See https://help.github.com/en/actions/reference/contexts-and-expression-syntax-for-github-actions#github-context
• The script will now exit early if there is nothing to commit which would otherwise result in a non-zero (failing) result. This shouldn't happen under normal circumstances, but good to add to prevent false failures.
  See https://github.com/google/site-kit-wp/runs/480649661?check_suite_focus=true
• Adds master to the branch whitelist for building and deploying zips to the wiki

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

[aaemnnosttv]

@adamsilverstein there seems to be an issue with commits not able to be pushed to the wiki repo now, which is odd because I'm sure this was working before. How did you have your PAT set before?

Left is the last run before the PR was merged (run) - Right is a recent run that failed (run)

remote: Permission to google/site-kit-wp.wiki.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/google/site-kit-wp.wiki/': The requested URL returned error: 403
##[error]Process completed with exit code 128.

According to the checkout action readme, it looks like we'll need to add a personal token back 😞

${{ github.token }} is scoped to the current repository, so if you want to checkout a different repository that is private you will need to provide your own PAT.

[adamsilverstein]

Looks good, thanks for adding this.