Fix host-mode checkout in Gitea workflow

Replace actions/checkout@v4 with a manual token-based clone — its files
were being copied into the act_runner workspace instead of the repo,
leaving nix to evaluate the wrong flake. Add a workspace inspection
step before the build to aid diagnosis.

.gitea/workflows/build.yml

@@ -9,7 +9,24 @@ jobs:
   nix-build:
     runs-on: build-server
     steps:
-      - uses: actions/checkout@v4
+      # actions/checkout@v4 is unreliable on host-mode runners (its files are
+      # copied into the workspace instead of the repo), so clone explicitly.
+      - name: Checkout
+        run: |
+          rm -rf ./* ./.[!.]* 2>/dev/null || true
+          git config --global http.extraheader "Authorization: token ${{ github.token }}"
+          git clone --depth 1 --branch "${{ gitea.ref_name }}" \
+            "${{ gitea.server_url }}/${{ gitea.repository }}.git" .
+          test -f flake.nix
+          git log --oneline -1
+
+      - name: Inspect workspace
+        run: |
+          pwd
+          ls -la
+          git rev-parse --show-toplevel 2>&1 || true
+          git remote -v 2>&1 || true
+          test -f flake.nix && echo "flake.nix present" || echo "NO flake.nix"
 
       - name: Build + offline test suite
         run: nix build -L .#checks.default