Nix: filter src robustly with gitignore.nix

[doronbehar]

I noticed this can be useful when playing with Nix files. The lack of sufficient VERGEN_* environment variables might tilt you towards not accepting it, but maybe I can think of different solutions - maybe without gitignore.nix.

[cassandracomar]

this makes it much harder to keep the package in-sync with what can be upstreamed to nixpkgs (PR pending). writing a real filter function for the source seems like a better idea.

[doronbehar]

this makes it much harder to keep the package in-sync with what can be upstreamed to nixpkgs (PR pending).

I had a very similar experience as here when I helped sile write their Nix files, and I'm not sure what exactly makes it harder.

writing a real filter function for the source seems like a better idea.

What isn't real with gitignore.nix? :)

[cassandracomar]

we can't use gitignore.nix in nixpkgs

[doronbehar]

we can't use gitignore.nix in nixpkgs

But of course you won't need to. If you are talking about the experience of modifying the nix/packages/default.nix file make it work for nixpkgs/pkgs/by-name/pi/pinnacle/package.nix, even now you have to replace the src arguments in the current expression with a real src = fetchFromGitHub. This PR barely makes that experience less comfortable.