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.

.github/workflows/build.yaml

@@ -78,19 +78,11 @@ jobs:
         uses: ./.github/actions/setup-nix
         with:
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-      - name: Install gnu sed
+      - name: Install nix-build-uncached
-        run: brew install gnu-sed
+        run: nix-env -f default.nix -iA nix-build-uncached
 
-      - name: Build everything
+      - name: Build everything (default.nix)
-        run: |
+        run: nix-build-uncached
-          # The --dry-run will give us a list of derivations to download from cachix and
-          # derivations to build. We only take those that would have to be built and then build
-          # those explicitly. This has the advantage that pure verification will not include
-          # a download anymore, making it much faster. If something needs to be built, only
-          # the dependencies required to do so will be downloaded, but not everything.
-          nix-build --dry-run 2>&1 \
-            | gsed -e '1,/derivations will be built:$/d' -e '/paths will be fetched/Q' \
-            | xargs nix-build
 
 
   stack:

default.nix

@@ -107,6 +107,9 @@ rec {
       inherit (pkgs.haskell.packages."${compiler}") ghcWithPackages;
     };
 
+  # Used by CI on MacOS
+  inherit (pkgs) nix-build-uncached;
+
   ### Tools
 
   cabalTools =