run private cache test only when secret is present

.github/workflows/test.yml

@@ -2,6 +2,8 @@ name: "cachix-action test"
 on:
   pull_request:
   push:
+    branches:
+      - master
 
 jobs:
   public-cache:
@@ -40,6 +42,7 @@ jobs:
     - run: nix-build test.nix
 
   private-cache:
+    if: ${{ github.ref == 'refs/heads/master' }}
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]