ci: Define tag to use custom runners
Conditionally include a file to add the tag 'lilypond' to all jobs so that they are assigned to our custom runners, but we can leave the shared runners enabled for external contributors forking the repository.
Conditionally include a file to add the tag 'lilypond' to all jobs so that they are assigned to our custom runners, but we can leave the shared runners enabled for external contributors forking the repository.