I see. Interesting idea with the green energy.
But basically a non-opensource CI would be very difficult to justify due to the vendor lock-in. Yet I'm using github-workflows out of convenience...
We don't care about vendor lock in, we can rewrite the build scripts. We care about support.
Not nearly as bad as writing a bunch of custom github actions or travis CI yaml files and needing to move.
You can replicate garnix's checks with much worse UX and less parallelism on any other service with a few lines of shell calling `nix flake check` and `nix build`. Making the UX good is left as an exercise for the reader.