Create check-flake.yml

added automatic `nix flake check` action on push.

.github/workflows/check-flake.yml

@@ -0,0 +1,20 @@
+name: Check flake
+
+on:
+  push:
+    branches:
+    - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - uses: cachix/install-nix-action@v20
+
+      - name: Check if flake.nix still evaluates
+        run: nix flake check