test nix master with cachix

.github/workflows/test.yml

@@ -1,9 +1,7 @@
 name: "cachix-action test"
 on:
   pull_request:
-  push:    
-    branches:
-      - master
+  push:
 
 jobs:
   public-cache:
@@ -76,3 +74,25 @@ jobs:
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
         installCommand: 'nix-env -j8 -f https://cachix.org/api/v1/install -iA cachix'
     - run: nix-build test.nix
+
+  nix-master:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install Nix
+      uses: cachix/install-nix-action@v13
+      with: 
+        nix_path: nixpkgs=channel:nixos-20.03
+        install_url: https://nixos-nix-install-tests.cachix.org/serve/lb41az54kzk6j12p81br4bczary7m145/install
+        install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+    - run: yarn install --frozen-lockfile
+    - run: yarn build
+    - name: Test public cache
+      uses: ./
+      with:
+        name: cachix-action
+        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - run: nix-build test.nix