Stale lock after network interruption.

[inkblot]

Transient network interruptions between the git server and the action runner that is running gh-action-mutex can result in a stale mutex lock which prevents subsequent jobs from running. The sequence that causes this outcome is when lock acquisition succeeds and in the post action, a transient network failure prevents a fetch on line 22 of utils.sh. A failure on this line does not produce any error message and the error code produced by the git command is swallowed by the || true at the end of the line, which lets the script proceed. Subsequent lines (utils.sh: 23, 113, 114) do produce error messages, but do not correctly identify the failure mode.