ci: replace nix-build-uncached with nix-fast-build

FIXME: this doesn't work yet since the cachix action doesn't support
flakes yet. See cachix/cachix-action#47

.github/workflows/cachix.yml

@@ -26,4 +26,4 @@ jobs:
       with:
         name: jakobkukla
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-    - run: nix shell nixpkgs#nix-build-uncached -c nix-build-uncached ./ci.nix
+    - run: nix run github:Mic92/nix-fast-build -- --skip-cached --no-nom --flake ".#nixosConfigurations.matebook.config.system.build.toplevel"

.github/workflows/update.yml

@@ -26,7 +26,7 @@ jobs:
         id: build
         run: |
           nix flake update
-          nix shell nixpkgs#nix-build-uncached -c nix-build-uncached ./ci.nix
+          nix run github:Mic92/nix-fast-build -- --skip-cached --no-nom --flake ".#nixosConfigurations.matebook.config.system.build.toplevel"
       - name: Commit flake.lock on success
         if: steps.build.outcome == 'success'
         uses: stefanzweifel/git-auto-commit-action@v5