Upstream changes needed for flakes

[domenkozar]

There are a few patches to flakes that should be upstreamed:

• Relax Nix language restrictions for flakes: this allows us to dynamically compute inputs from yaml files.
• --flake-file and --flake-lock parameters to override flake.nix and flake.lock: this allows us to dynamically compute flakes and hide it from the users
• Allow reading files that aren't managed by git: this allows us to dynamically compute flake files so they aren't committed to a repo.
• Speed up GC
• abstract source trees, from Eelco's talk at NixCon, this one speeds up flakes by a lot.