gitlab-runner service: support graceful termination (#27222)

The current behavior was for gitlab-runner is to immediately terminate when there
was a restart required. This can lead to aborted builds and is annoying to users.

By enabling graceful mode gitlab-runner will wait for all builds to finish before
terminating. The disadvantage is that a nixos-rebuild switch needs to wait till
all jobs are done. Because of that it is not enabled by default.