github-runners: adapt to NixOS module

While #859 added basic support for configuring GitHub runners through
nix-darwin, it did not yet support all of the options the NixOS module
offers.

I am aware that this is a rather big overhaul. I think, however, that
it's worth it:

- Copies the `options.nix` from the [NixOS module] with only minor
  adaptations. This should help to keep track of any changes to it.
- Respect the `workDir` config option. So far, the implementation didn't
  even read the value of the option.
- Allow configuring a custom user and group.
  If both are `null`, nix-darwin manages the `_github-runner` user
  shared among all instances. Take care of creating your own users if
  that's not what you want.
- Also creates the necessary directories for state, logs and the working
  directory (unless `workDir != null`). It uses the following locations:
    * state: `/var/lib/github-runners/${name}`
    * logs: `/var/log/github-runners/${name}`
    * work: The value of `workDir` or `/var/run/github-runners/${name}`
            if (`workDir == null`).
  We have to create the logs directory before starting the service since
  launchd expects that the `Standard{Error,Out}Path` exist. We do this
  by prepending to [`system.activationScripts.launchd.text`].
  All directories belong to the configured `user` and `group`.
- Warn if a `tokenFile` points to the Nix store.

[NixOS module]: https://github.com/NixOS/nixpkgs/blob/3c30c56/nixos/modules/services/continuous-integration/github-runner/options.nix
[`system.activationScripts.launchd.text`]: https://github.com/LnL7/nix-darwin/blob/bbde06b/modules/system/launchd.nix#L99-L123

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