fix: explicitly checkout v3-alpha branch in nightly workflow

Ensures the workflow always checks out the v3-alpha branch regardless
of which branch triggers it (schedule from master or manual dispatch)

.github/workflows/nightly-release-v3.yml

@@ -29,6 +29,7 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v4
       with:
+        ref: v3-alpha
         fetch-depth: 0
         token: ${{ secrets.GITHUB_TOKEN }}