[fix] unbreak the nix code and make it slightly nicer

[MangoIV]

Fixes

• [x] dex builds
• [x] dex-cuda builds
• [x] dex tests pass
• [ ] dex-cuda tests pass (see Warning, I don't have an nvidia graphics card)

Additions

• nix formatter can be run with nix fmt
• replaced old devShell and package argument with <output>.<system>.default
• don't build the dex runtime in the configurePhase but instead build it as a separate drv
• made a couple of things more idiomatic

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Note the packages of llvm-hs, llvm-hs-pure and the dex package should probably be materalized, I'll perhaps at this as a PR at a later point

at some point we should probably make the legacy nix code properly follow the flake inputs I didn't update this but as nixpkgs is not pinned anyways, I don't think it makes sense to half-ass it.

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Warning before merging this, someone with an nvidia graphics card should probably check whether this builds by running nix build github:mangoiv/dex-lang#dex-cuda on their system with an nvidia graphics card and ld library path properly set

be aware that dex does not build with current nixpkgs-unstable as floating-bits is broken for a while, similarly to llvm-hs/*

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[MangoIV]

I don't have a personal google account and I won't create one; I will contribute under usual terms but if that's not possible I perhaps cannot.