Use "nix develop -c cabal v2-build -O0" to speed up the workflow

With "nix build", just the build of xmonad-contrib itself takes 3
minutes (it builds twice, the second build with profiling enabled), so
it ends up being almost 6 minutes in total, making this workflow the
slowest one.

.github/workflows/nix.yml

@@ -22,4 +22,7 @@ jobs:
       - name: Clone project
         uses: actions/checkout@v2
       - name: Build
-        run: nix build --print-build-logs
+        # "nix build" builds with full optimization and includes a profiling
+        # build, so just the build of xmonad-contrib itself takes 3 minutes.
+        # As a workaround, we invoke cabal manually here.
+        run: nix develop -c cabal v2-build -O0 -j

flake.nix

@@ -23,6 +23,7 @@
   rec {
     devShell = pkgs.haskellPackages.shellFor {
       packages = p: [ p.xmonad-contrib ];
+      nativeBuildInputs = [ pkgs.cabal-install ];
     };
     defaultPackage = pkgs.haskellPackages.xmonad-contrib;
   }) // { inherit overlay overlays; } ;