releng-audit: clear state labels and set pending status on comment triggers

When /audit retest or /audit test-branch is invoked via an issue_comment event, GitHub Actions does not natively bind the execution to the PR's HEAD commit SHA. As a result, the check widget on the PR conversation tab remains in its previous state (often showing as failed), and any lingering releng-audit-fail or releng-audit-pass labels remain attached.

This introduces a triggerAuditRun helper function to the workflow router that:

- Removes any existing releng-audit-fail or releng-audit-pass labels from the pull request to reset the label state machine and prevent subsequent synchronize events from halting prematurely.
- Explicitly creates a pending commit status on the PR's HEAD SHA via the GitHub REST API, ensuring visual consistency and feedback inside the PR UI while manual or test-branch audits execute in the background.

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