add flake check

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/check.yml

@@ -0,0 +1,16 @@
+name: "Check flake"
+
+on:
+  push:
+    paths:
+      - "**.nix"
+      - "**.lock"
+      - ".github/workflows/check.yml"
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v22
+      - run: nix flake check --show-trace --accept-flake-config