ci: use nix-build-uncached to save useless downloads

There is no point to actually realize the derivations to disk, when they
are in cache. `nix-build-uncached` checks the cache and stops before
downloading dependencies if they are available. It will however build as
normal on a cache-miss. Should speed up CI when no Nix dependencies
change a fair bit.

We can only do this for the MacOS job, which only tests the builds. We
can not do this for other jobs which require the build results to be
present before uploading parts of them as artifacts.

default.nix

@@ -108,6 +108,9 @@ rec {
       inherit (pkgs.haskell.packages."${compiler}") ghcWithPackages;
     };
 
+  # Used by CI on MacOS
+  inherit (pkgs) nix-build-uncached;
+
   ### Tools
 
   cabalTools =