Merge pull request #57 from nix-community/ci

fix nix 2.6 fallout

.github/workflows/build.yml

@@ -36,7 +36,7 @@ jobs:
         nixPath:
           - nixpkgs=channel:nixos-unstable
           - nixpkgs=channel:nixpkgs-unstable
-          - nixpkgs=channel:nixos-21.05
+          - nixpkgs=channel:nixos-21.11
     runs-on: ubuntu-latest
     steps:
     - name: Checkout repository
@@ -45,6 +45,9 @@ jobs:
       uses: cachix/install-nix-action@v16
       with:
         nix_path: "${{ matrix.nixPath }}"
+        # nix 2.6 breaks restrict-eval, when using the NIX_PATH
+        # see https://github.com/NixOS/nix/issues/5980
+        install_url: https://releases.nixos.org/nix/nix-2.5.1/install
         extra_nix_config: |
           experimental-features = nix-command flakes
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}