[ci] Schedule Coverity Scan run via GitHub Actions

Trigger the daily Coverity Scan submission via a GitHub Actions
schedule (or via a manual workflow run), rather than relying on an
external process pushing to the "coverity_scan" branch.

Since the scheduled workflow will run even on forks of the repository,
add a check to cause the submission to be skipped if the relevant
secret is not configured.

Signed-off-by: Michael Brown <mcb30@ipxe.org>