Fix cache-manager errexit on _check_stale_lock return

The _check_stale_lock function returns 1 when the lock is not stale (normal case).
With set -euo pipefail, calling it directly (not in a conditional) causes the
script to exit immediately, preventing NFS cache push.

Add '|| true' to ignore the return value since we only care about the side
effect of cleaning up stale locks.