Nix language support improvements, including smaller Cachix

[therealpxc]

This fix saves 7 GB in the closure size of my devenv projects which have Nix language support enabled.

Background: Currently, with languages.nix.enable = true;, all outputs of the cachix package in Nixpkgs get installed. Unlike the Cachix package provided by the cachix/cachix flake, this is a massive multi-output derivation that includes a huge Haskell runtime with libs in .out and more than a gigabyte of library documentation in .doc.

The original issue here raises some questions for me about how devenv profiles are constructed and whether they do or should honor meta.outputsToInstall-- maybe there's something to think about there, too. (The Cachix package in nixpkgs has only "bin" in meta.outputsToInstall.)

Anyway, this fix works now and is a huge quality of life improvement for me!

[domenkozar]

Maybe we should just use devenv.bin?

[therealpxc]

@domenkozar you wrote:

Maybe we should just use devenv.bin?

You mean cachix.bin? Devenv doesn't have a bin output either in Nixpkgs or our flake.

At any rate, that's essentially all we do here: lib.getBin will get the bin output of the provided Cachix derivation, unless it doesn't have one. If it doesn't have one, it'll fall back to cachix.out. This makes the Cachix module compatible both with Cachix from Nixpkgs and Cachix from the github:cachix/cachix flake, which I suppose might be nice if you want old Nixpkgs but new Cachix.

We use string interpolation because lib.getBin actually produces a derivation with some magic that makes it yield the bin output when coerced into a string. Functions that expect derivations will thus end up getting the same old .out as always. The same thing happens when naively putting just cachix.bin in for the package-- you end up with cachix.out and cachix.doc alongside cachix.bin. But interpolate it in a string and you get just the bin output as expected.