Nix: evaluate the flake in CI so a new git dependency can't break nix build / nix develop again

Since UI.9 (9bfabc98) Cargo.lock has a git source, craft-ui from
storytold/craft-libs. importCargoLock needs a hash for every git
dependency, so evaluation stopped with "No hash was found while vendoring
the git dependency craft-ui-0.1.0" and both `nix build` and `nix develop`
failed on NixOS for two days (#1314 fixed it with
cargoLock.allowBuiltinFetchGit).

The flake is not built in CI, so nothing noticed: the new Nix workflow
runs `nix flake check --no-build --all-systems` whenever flake.nix,
flake.lock, nix/ or Cargo.lock change.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/nix.yml

@@ -0,0 +1,28 @@
+# Evaluates the Nix flake (no build, about a minute): `nix build` / `nix develop` stop at evaluation
+# when Cargo.lock gains something nix/package.nix can't vendor, and nothing else would notice.
+name: Nix
+
+on:
+  push:
+    branches: [main]
+    paths: ["flake.nix", "flake.lock", "nix/**", "Cargo.lock", ".github/workflows/nix.yml"]
+  pull_request:
+    paths: ["flake.nix", "flake.lock", "nix/**", "Cargo.lock", ".github/workflows/nix.yml"]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: nix-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  eval:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: cachix/install-nix-action@13d8dd58da0234aa297dedd986986ccb8e7f3e24 # v31.11.1
+      - name: nix flake check --no-build
+        run: nix flake check --no-build --all-systems