CI: GitHub: workflow: add project Nixpkgs-integration test

.github/workflows/On-Release-Nixpkgs-integration.yml

@@ -0,0 +1,27 @@
+name: "Direct import & build inside Nixpkgs"
+
+on:
+  release:
+    # created: a draft is saved, or a release or pre-release is published without previously being saved as a draft
+    types: [ created ]
+
+jobs:
+
+  build10:
+    name: "Clean integration test"
+    runs-on: ubuntu-latest
+    steps:
+    - name: "Git checkout"
+      uses: actions/checkout@v2
+    - name: "Local cache"
+      uses: actions/cache@v2
+      with:
+        path: |
+          /nix/store
+        key: ${{ runner.os }}-Nixpkgs-integration-test
+    - name: "Install Nix"
+      uses: cachix/install-nix-action@v12
+      with:
+        nix_path: "nixpkgs=channel:nixos-unstable"
+    - name: "Run Nixpkgs integration test"
+      uses: Anton-Latukha/test-haskell-nixpkgs-integration-action@v1