ci,nix: added nix flake check to ci

Author: lromor

.github/workflows/ci.yml

@@ -89,6 +89,15 @@ jobs:
           clang-tidy-18 --version
           CLANG_TIDY=clang-tidy-18 scripts/run-clang-tidy-cached.cc \
             || ( cat fpga-assembler_clang-tidy.out ; exit 1)
+  NixCheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v25
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - run: |
+          nix flake check
   NixBuild:
     runs-on: ubuntu-latest
     steps: