Fix flakes example and add a test

.github/workflows/test.yml

@@ -58,3 +58,23 @@ jobs:
           sandbox = relaxed
     - run: cat /etc/nix/nix.conf
     - run: nix-build test.nix --arg noChroot true
+
+  flakes:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        # Nix Flakes doesn't work on shallow clones
+        fetch-depth: 0
+    - run: yarn install --frozen-lockfile
+    - run: yarn build
+    - name: Install Nix
+      uses: ./
+      with:
+        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20201221_9fab14a/install
+        extra_nix_config: |
+          experimental-features = nix-command flakes
+    - run: nix flake show github:NixOS/nixpkgs

README.md

@@ -59,7 +59,7 @@ jobs:
           fetch-depth: 0
     - uses: cachix/install-nix-action@v11
       with:
-        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-3.0pre20200820_4d77513/install
+        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-3.0pre20201007_5257a25/install
         extra_nix_config: |
           experimental-features = nix-command flakes
     - run: nix-build