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

@@ -17,7 +17,6 @@ or [pin nixpkgs yourself](https://nix.dev/reference/pinning-nixpkgs.html)
 - Allows specifying Nix installation URL via `install_url`
 - Allows specifying extra Nix configration options via `extra_nix_config`
 - Allows specifying `$NIX_PATH` and channels via `nix_path`
-- Share `/nix/store` between builds using [cachix-action](https://github.com/cachix/cachix-action) for simple binary cache setup to speed up your builds and share binaries with your team
 
 ## Usage
 
@@ -39,6 +38,9 @@ jobs:
     - run: nix-build
 ```
 
+See also [cachix-action](https://github.com/cachix/cachix-action) for
+simple binary cache setup to speed up your builds and share binaries
+with developers.
 
 ## Usage with Flakes
 
@@ -57,7 +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