Allow use of a customised systemd template in svc.sh

[andrewbelcher]

Describe the enhancement
https://docs.github.com/en/actions/hosting-your-own-runners/configuring-the-self-hosted-runner-application-as-a-service#customizing-the-self-hosted-runner-service describes the use of customised services and recommend using actions-runner/bin/actions.runner.service.template as a reference. But to do that, you have to completely manage the service yourself.

It would be really handy if there was an optional argument for svc.sh that allowed you to specify an alternative template file, but still have svc.sh handle replacements and be able to manage the service.

[ethomson]

Thanks for the suggestion @andrewbelcher - this seems like a reasonable addition. I'm adding the good-first-issue tag in case somebody's interested in creating a PR.

[nikola-jokic]

#1612

[TingluoHuang]

The change will be included in the next runner release. 🙇