port to flakes

.github/workflows/nix-flake-check.yml

@@ -0,0 +1,15 @@
+name: "nix flake check"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v13
+        with:
+          install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+      - run: nix flake check