ci: faster flake checks... maybe?

Signed-off-by: Christina Sørensen <ces@fem.gg>

.github/workflows/unit-tests.yml

@@ -64,6 +64,10 @@ jobs:
     needs: conventional
     name: Check Nix Flake
     runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        checks: [build, formatting, lint, pre-commit-check, test, trycmd]
+        target: [x86_64-linux]
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
@@ -71,7 +75,7 @@ jobs:
       - name: Setup Nix cache
         uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Nix Flake Check
-        run: nix flake check --all-systems -L
+        run: nix build .#packages.${{ matrix.target }}.${{ matrix.checks }} -L
 
   flake-build:
     needs: conventional