[devtools] Add GitHub Action for Nix build

.github/workflows/nix-toil-run-cpp.yml

@@ -0,0 +1,21 @@
+# (based on: https://github.com/cachix/install-nix-action readme)
+name: "(Nix) toil run-cpp"
+on:
+  pull_request:
+  push:
+jobs:
+  toil-run-cpp:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        # Nix Flakes doesn't work on shallow clones
+        fetch-depth: 0
+    - uses: cachix/install-nix-action@v12
+      with:
+        nix_path: nixpkgs=channel:nixos-20.09
+        # Enable Nix Flakes
+        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 build -L .#toil-run-cpp