add flake check workflow

.github/workflows/check.yml

@@ -0,0 +1,25 @@
+name: check
+on:
+  workflow_dispatch:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Install Nix
+        uses: cachix/install-nix-action@v30
+        with:
+          enable_kvm: true
+          extra_nix_config: |
+            show-trace = true
+            experimental-features = nix-command flakes
+            extra-substituters = https://nixcache.cy7.sh
+            extra-trusted-public-keys = nixcache.cy7.sh:DN3d1dt0wnXfTH03oVmTee4KgmdNdB0NY3SuzA8Fwx8=
+
+      - uses: actions/checkout@v4
+
+      - name: Run checks
+        run: nix flake check -L