.github/workflows: Improve pipeline with some parallel steps

In order to speed up the CI pipeline, we are parallelizing the builds. This does unfortunately re-perform a lot of the tasks, specifically the disk space free, which is arguably the second most lengthy step after the build itself. I cannot find out how to avoid this so at the moment this will be the best change we can do. If there are ways to "snapshot" an action's state and allow parallelization from that snapshot forwards, that could be cool. I know a VM/Container could do this, but I've no idea how to do that using GitHub Actions yaml logic.

.github/workflows/build.yml

@@ -7,28 +7,32 @@ on:
       - staging
 
 jobs:
-  tests:
+  test:
+    name: Test
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        outputs: [
+          'nixosConfigurations.LAPTOP-3DT4F02.config.system.build.toplevel',
+          'nixosConfigurations.DESKTOP-3DT4F02.config.system.build.toplevel',
+          'nixosConfigurations.ISO-3DT4F02.config.system.build.isoImage'
+        ]
     steps:
-      - uses: actions/checkout@v4.2.2
-
       - name: Free Disk Space
         uses: jlumbroso/free-disk-space@v1.3.1
-        with:
-          tool-cache: true
 
+      - name: Checkout Repository
+        uses: actions/checkout@v4.2.2
+
       - name: Install Nix
         uses: cachix/install-nix-action@v30
         with:
           extra_nix_config: |
             allow-import-from-derivation = false
             eval-cache = false
 
-      - name: Run Flake Check
-        run: nix flake check
+      - run:
+          nix flake check
 
-      - name: Build NixOS Configurations
-        run: |
-          nix build '.#nixosConfigurations.LAPTOP-3DT4F02.config.system.build.toplevel'
-          nix build '.#nixosConfigurations.DESKTOP-3DT4F02.config.system.build.toplevel'
-          nix build '.#nixosConfigurations.ISO-3DT4F02.config.system.build.isoImage'
+      - run:
+          nix build '.#${{ matrix.outputs }}'