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

@@ -22,10 +22,8 @@ jobs:
       with:
         github_access_token: ${{ secrets.GITHUB_TOKEN }}
     - uses: DeterminateSystems/magic-nix-cache-action@main
-    - name: Install nix-build-uncached
-      run: nix-env -f '<nixpkgs>' -iA nix-build-uncached
     - uses: cachix/cachix-action@v14
       with:
         name: jakobkukla
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
-    - run: 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

@@ -18,8 +18,6 @@ jobs:
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
       - uses: DeterminateSystems/magic-nix-cache-action@main
-      - name: Install nix-build-uncached
-        run: nix-env -f '<nixpkgs>' -iA nix-build-uncached
       - uses: cachix/cachix-action@v14
         with:
           name: jakobkukla
@@ -28,7 +26,7 @@ jobs:
         id: build
         run: |
           nix flake update
-          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