fix: replace actions/checkout with native git clone to remove GitHub dependency

The workflow previously used actions/checkout@v4 which fetches from GitHub.
Gitea runner cannot access GitHub due to timeout. Replace with inline
git init + fetch using GITHUB_SERVER_URL / GITHUB_REPOSITORY env vars
provided by Gitea Actions runtime.

.gitea/workflows/wheel-pipeline.yaml

@@ -17,7 +17,11 @@ jobs:
     steps:
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        run: |
+          git init
+          git remote add origin "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}.git"
+          git fetch --depth 1 origin "${GITHUB_SHA}"
+          git checkout "${GITHUB_SHA}"
 
       - name: Show environment info
         run: |