ci: Add check for evaluation only

Makes it easier to see if evaluation passes but end-to-end tests fail.

.github/workflows/tests.yml

@@ -7,6 +7,13 @@ on:
     branches: [ master ]
 
 jobs:
+  check:
+    name: nix flake check --no-build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v22
+      - run: nix flake check --no-build
   nix:
     runs-on: ubuntu-latest
     steps: