build-3rdparty.yml: Fix repo branch name

${{ env.GITHUB_HEAD_REF }} is not available to workflow_dispatch [1],
so the branch name must be manually extracted from
${{ env.GITHUB_EVENT_PATH }}.

[1]: https://forgejo.org/docs/latest/user/actions/#env-1

.forgejo/workflows/build-3rdparty.yml

@@ -12,8 +12,9 @@ jobs:
     steps:
       - name: Clone repository
         run: |
+          branch=$(jq -r '.ref | ltrimstr("refs/heads/")' ${{ env.GITHUB_EVENT_PATH }})
           git clone -q \
-            -b ${{ env.GITHUB_HEAD_REF }} \
+            -b $branch \
             ${{ env.GITHUB_SERVER_URL }}/${{ env.GITHUB_REPOSITORY }}.git \
             /src
       - name: Configure third-party libraries