.github/workflows/releng-audit: support testing via alternate branch

.github/workflows/releng-audit: support testing via alternate branch

Allow authorized maintainers and release managers to test audit workflow
changes on a pull request using an alternate repository branch.

When a user comments `/audit test-branch [branch-name]` on a PR:
* Verify user authorization and confirm the target branch exists via the
  GitHub API (defaulting to `testing/releng-audit` if unspecified).
* If valid, activate audit execution and set the `checkout_ref` output
  to dynamically override the repository checkout target.
* If the branch does not exist or the user lacks permissions, fail the
  job cleanly and leave an explanatory comment.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>