Support self-hosted runners and always skip adding a channel

.github/workflows/test.yml

@@ -15,28 +15,14 @@ jobs:
     - run: yarn build
     - name: Install Nix
       uses: ./
+      with:
+        nix_path: nixpkgs=channel:nixos-20.03
     - run: nix-env -iA cachix -f https://cachix.org/api/v1/install
     - run: cat /etc/nix/nix.conf
     # cachix should be available and be able to configure a cache
     - run: cachix use cachix
     - run: nix-build test.nix
 
-  no-channel:
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest]
-    runs-on: ${{ matrix.os }}
-    steps:
-    - uses: actions/checkout@v2
-    - run: yarn install --frozen-lockfile
-    - run: yarn build
-    - name: Install Nix
-      uses: ./
-      with:
-        skip_adding_nixpkgs_channel: true
-    - run: nix-build test.nix && exit 1 || echo "OK"
-    - run: NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/tarball/ab5863afada3c1b50fc43bf774b75ea71b287cde nix-build test.nix
-
   custom-nix-path:
     strategy:
       matrix:
@@ -64,7 +50,8 @@ jobs:
     - run: yarn build
     - name: Install Nix
       uses: ./
-      with:
+      with: 
+        nix_path: nixpkgs=channel:nixos-20.03
         extra_nix_config: |
           sandbox = relaxed
     - run: cat /etc/nix/nix.conf