`guix build -f` currently isn't supported. For now run `guix environment -l guix.scm` and build from source that way. Instructions are in the README. I can try to make guix.scm a true buildable package soon.
Hacking guix.scm for an ad-hoc environment felt like a better idea at the time, so I did that. On a somewhat related note, do you really need to patch Guile itself? My gut instinct tells me you should be able to ship a patched GOOPS with starling similarly to how I used a "magic" module to introduce pre-release Guile-SDL2 functionality to Tsukundere.
I suppose I could include a modified copy of goops.scm and take advantage of load order to have my version loaded. Might try that later. At the time I wanted to test a full Guile build since I submitted the patch upstream and wanted a true integration test, but the patch got merged awhile ago so it's no longer necessary. Thanks for the suggestion.