The `serviceOverrides` module option is commonly used to loosen the
systemd unit's hardening. This commit merges the `serviceConfig` with
`mkMerge` instead of using the update operator `//` which discards all
existing values on conflict. To avoid a breaking change which requires
defining each option with a higher priority (e.g., through `mkForce`),
this commit prefixes hardening values with `mkDefault`.
Notable exceptions are list hardening options which use `mkBefore`
instead of `mkDefault`. This allows for easy extension of the existing
settings. Resetting redefinitions are still possible through `mkForce`.
On some occasions, the GitHub runner service encounters errors which are
deemed retryable but result in the runner's termination. To signal a
retryable error, the runner exits with status code 2:
https://github.com/actions/runner/blob/40ed7f8/src/Runner.Common/Constants.cs#L146
To account for that behavior, this commit sets
`RestartForceExitStatus=2` which results in a service restart regardless
of using an ephemeral runner or not.
