Add 'if: startsWith(runner.name, "runs-on--")' to the mirror swap step
in both workflows so fork users can see at a glance that the step only
fires on runs-on runners and is a no-op on standard GitHub-hosted
runners. The script keeps its internal RUNS_ON_AWS_REGION check as
defense in depth for callers outside these workflows.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>