add nix build github workflow

.github/workflows/nix-build.yaml

@@ -0,0 +1,22 @@
+name: nix-build
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  nix-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2.3.4
+      - uses: cachix/install-nix-action@v13
+        with:
+          install_url: https://nixos-nix-install-tests.cachix.org/serve/lb41az54kzk6j12p81br4bczary7m145/install
+          install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+      - run: nix develop -c rustc --version
+      - run: nix run . -- --version
+      - run: nix-build

.github/workflows/tests.yaml

@@ -1,6 +1,7 @@
 name: tests
 
 on:
+  pull_request:
   push:
     branches:
       - master