ci: replace actions/checkout with manual clone

actions/checkout@v4 on a subpath-mounted gitea (ROOT_URL=
https://famzheng.me/gitea/) resolves $github.server_url to
https://famzheng.me/ (without /gitea/), so the clone fetches
https://famzheng.me/fam/nanochat-omni and 404s. Clone manually
against the full URL.

.gitea/workflows/smoke.yml

@@ -8,7 +8,14 @@ jobs:
     runs-on: gpu
     timeout-minutes: 30
     steps:
-      - uses: actions/checkout@v4
+      - name: checkout
+        # actions/checkout@v4 mis-resolves server_url on subpath gitea
+        # (drops /gitea/ prefix), so clone manually.
+        run: |
+          git init .
+          git remote add origin https://famzheng.me/gitea/${{ github.repository }}.git
+          git fetch --depth=1 origin ${{ github.sha }}
+          git checkout FETCH_HEAD
       - name: nvidia-smi
         run: nvidia-smi --query-gpu=name,memory.free,memory.used --format=csv
       - name: smoke