Fix 'cleanSource' function, re-add nixpkgs Nix formatter.

[swagtop]

The problem with the current cleanSource

Currently the cleanSource function grabs any files matching the list of regular expressions. In flakes, these files will be put in a new derivation, with a name based on the NAR info of the flake that has been evaluated.

This means that any changes to anything that is tracked by git, will result in a src input with a new name, triggering a full rebuild of anything that uses ./. as a source, even if all of the files found through the regex are the exact same as the previous derivation with a different name.

To test this, run nix build, then change one character in any file tracked by git in the repo, and run nix build again.

The solution

Use the builtins.path function to import the path, set a name (I've arbitrarily chodsen futhark-${commit}-src), and use the regex from the old cleanSource in the filter attribute. The result is that any changes to anything that does not relate to the actual build will have no impact on the build, and unnecessary rebuilds are avoided.

The code added is lifted from the nixpkgs lib, and edited to match the existing cleanSource function. Therefore it should be super robust, and unlikely to ever break.

I have built every package in the repo, and everything builds successfully, works, and it resilient to being needlessly rebuilt on arbitrary changes.

Other changes

I have re-added the nixpkgs Nix formatter as formatter in the flake output. It only takes up one line of code, and will be immensely useful for making changes to the Nix code in the future.

[athas]

The Git integration is probably my least favourite part of Flakes.

[swagtop]

The Git integration is probably my least favourite part of Flakes.

I used to feel the same but I hate to say that I have come to love it a lot...

[swagtop]

Also, another change that could be considered, that I did not include in this PR, is that there is the attribute self.dirtyRev that one can fall back on between self.rev and "unknown", when setting the commit attribute.

The output of this would be the latest commit rev, with a -dirty suffix on it. It could give some info that was a little more specific than "unknown". The implementation would look like: commit = self.rev or self.dirtyRev or "unknown".

[athas]

To clarify, I don't mind this change, but nix build is not part of the actual Futhark development workflow, so it doesn't matter how sensitive it is to small changes.