actions: add ci

.github/workflows/check.yml

@@ -0,0 +1,19 @@
+name: Check flake (run tests)
+
+on:
+  push:
+
+jobs:
+  check:
+    strategy:
+      max-parallel: 1
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          ref: 'master'
+      - uses: cachix/install-nix-action@v22
+        with:
+          # explicitly enable sandbox
+          extra_nix_config: sandbox = true
+      - run: nix flake check