chore: making `nixpkgs-matrix` an indirect input

[tegefaulkes]

Description

This PR incorporates the new indirect flake input system.

Tasks

• [x] 1. Update flake.nix to use indirect inputs

Final checklist

• [x] Domain specific tests
• [x] Full tests
• [x] Updated inline-comment documentation
• [x] Lint fixed
• [x] Squash and rebased
• [x] Sanity check the final build

[linear[bot]]

INF-79 Update all project repos to use indirect flake input to nixpkgs-matrix or nixpkgs-matrix-private

[tegefaulkes]

I'm doing this as a PR here for reference and just so that @brynblack can check I got the change right.

[tegefaulkes]

Ah, I can't seem to find nixpkgs-matrix in the flake registries. I possibly have to do a nix rebuild to get that into my profile right?

[tegefaulkes]

Ok, I only had to add it to the registry with nix registry add nixpkgs-matrix github:MatrixAI/nixpkgs-matrix.

[brynblack]

This looks good. I think it can be merged once we have all our other repos ready for the transition.