build: use explicitly generated Cargo.nix instead of ifd

Author: Defelo

.gitattributes

@@ -0,0 +1 @@
+Cargo.nix linguist-vendored

.github/workflows/check.yml

@@ -48,3 +48,12 @@ jobs:
         uses: actions-rs/clippy-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
+
+  cargo-nix:
+    name: cargo-nix
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+      - run: nix run .#generate
+      - run: git diff --exit-code