github actions: test build from flake

.github/workflows/check-build.yml

@@ -0,0 +1,21 @@
+name: "Test"
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - develop
+jobs:
+  tests:
+    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:
+        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 flake check -L