Add job: nixpkgs-fmt. Parametrize nix_path from env

Author: rszamszur

template/{% if github_actions %}.github{% endif %}/workflows/{% if nix %}nix.yml{% endif %}.jinja

@@ -9,6 +9,9 @@ on:
     branches:
       - master
 
+env:
+  NIX_CHANNEL: nixpkgs=channel:nixos-22.05
+
 jobs:
   # This job checks if an identical workflow is being triggered by different
   # event and skips it. For instance there is no need to run the same pipeline
@@ -24,6 +27,18 @@ jobs:
           skip_after_successful_duplicate: 'true'
           concurrent_skipping: same_content
           do_not_skip: '["pull_request", "workflow_dispatch", "schedule"]'
+  nixpkgs-fmt:
+    needs: pre_job
+    if: ${{ needs.pre_job.outputs.should_skip != 'true' }}
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: cachix/install-nix-action@v17
+        with:
+          nix_path: ${{ env.NIX_CHANNEL }}
+      - uses: actions/checkout@v3
+      - name: Check format
+        run: nix-shell -p nixpkgs-fmt --run 'nixpkgs-fmt --check .'
   nix-build:
     needs: pre_job
     if: ${{ needs.pre_job.outputs.should_skip != 'true' }}
@@ -35,7 +50,7 @@ jobs:
         uses: cachix/install-nix-action@v17
         with:
           extra_nix_config: "system-features = nixos-test benchmark big-parallel kvm"
-          nix_path: nixpkgs=channel:nixos-22.05
+          nix_path: ${{ env.NIX_CHANNEL }}
       # Remove bellow step if you do not want to use Cachix - Nix binary cache.
       # For OpenSource projects there is free 5GB of storage.
       # https://www.cachix.org
@@ -66,7 +81,7 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v17
         with:
-          nix_path: nixpkgs=channel:nixos-22.05
+          nix_path: ${{ env.NIX_CHANNEL }}
       # Remove bellow step if you do not want to use Cachix - Nix binary cache.
       # For OpenSource projects there is free 5GB of storage.
       # https://www.cachix.org