do not run macos every time

.github/workflows/check.yml

@@ -1,5 +1,6 @@
 name: "Test template"
 on:
+  workflow_dispatch: # allows manual triggering
   pull_request:
   push:
 jobs:
@@ -7,11 +8,12 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest]
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v31
       with:
         github_access_token: ${{ secrets.GITHUB_TOKEN }}
+    - uses: DeterminateSystems/magic-nix-cache-action@main
     - run: nix build ./templates/n
     - run: nix flake check ./templates/n