ci: run nix flake check on ci (#583)

* ci: run nix flake check on ci

* ci: fix tests

.github/workflows/nix.yml

@@ -0,0 +1,35 @@
+# Copyright © 2025-26 l5yth & contributors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: Nix
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  flake-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: cachix/install-nix-action@v30
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+      - name: Run flake checks
+        run: nix flake check