nix: move ci dependencies to the devShell

[TristanCacqueray]

Fixes #160

[zimbatm]

 = note: ld: library not found for -liconv
          clang-7: error: linker command failed with exit code 1 (use -v to see invocation)

I'm not sure why this is failing on macOS

[TristanCacqueray]

@zimbatm shouldn't the override appends to the treefmt buildInputs? e.g. to keep that definition:

treefmt/default.nix

Line 49 in e7918e9

buildInputs = with nixpkgs; lib.optionals stdenv.isDarwin [ darwin.apple_sdk.frameworks.Security libiconv ];

[zimbatm]

doh. that's probably it :)

[zimbatm]

@TristanCacqueray nativeBuildInputs probably also needs to be concatenated with prev.nativeBuildInputs

[TristanCacqueray]

@zimbatm oh, ok I've squashed the change. We'll get through this eventually :)

[zimbatm]

success!

[TristanCacqueray]

nice, thanks!