Updated workflow - NOTE: CACHIX_AUTH_TOKEN env is now required

.github/workflows/build_with_nix.yml

@@ -1,8 +1,8 @@
 name: build with nix
 on:
-  schedule:
-    # 02:00 every Sunday morning
-    - cron: '0 2 * * 0'
+  pull_request:
+    branches:
+      - main
   workflow_dispatch: {}
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -11,10 +11,14 @@ jobs:
   build_and_test:
     strategy:
       matrix:
-        os: ['ubuntu-22.04', 'macos-13-xlarge']
+        os: ['ubuntu-22.04', 'macos-14']
     runs-on: ${{matrix.os}}
     steps:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v25
-    - name: Build and test tket
+    - uses: cachix/cachix-action@15
+      with:
+        name: tket
+        signingKey: ${{ secrets.CACHIX_AUTH_TOKEN }}
+    - name: Build and test pytket-qir
       run: nix flake check -L