I get the error from the title when running this locally. Not sure how it's possible, given the source code has a clear "github" case right there...
mbp23:tmp user$ git clone https://github.com/Mic92/flake-linter
Cloning into 'flake-linter'...
remote: Enumerating objects: 16, done.
remote: Counting objects: 100% (16/16), done.
remote: Compressing objects: 100% (13/13), done.
Receiving objects: 100% (16/16), 6.00 KiB | 6.00 MiB/s, done.
Resolving deltas: 100% (1/1), done.
remote: Total 16 (delta 1), reused 12 (delta 1), pack-reused 0
mbp23:tmp user$ cd flake-linter/
mbp23:flake-linter user$ nix run github:Mic92/flake-linter
Unknown input type github
Unknown input type github
Unknown input type github
mac multi user, not sure what other details can be useful here. If you have a debug version I"m happy to try it.