Empty the work directory instead of removing it

It is a bind mount since the job container gained the host volume, and
removing a mount point fails with "Device or resource busy".

Claude-Session: https://claude.ai/code/session_01GGe7kUhooMFuzY6GaDb2w9

.gitea/workflows/build.yml

@@ -78,7 +78,8 @@ jobs:
 
       - name: Prepare work directory
         run: |
-          sudo rm -rf "${WORK_DIR}"
+          # WORK_DIR is a bind mount now, so empty it rather than removing it.
+          sudo find "${WORK_DIR}" -mindepth 1 -delete
           sudo mkdir -p "${WORK_DIR}/out"
           sudo cp -a build packaging scripts "${WORK_DIR}/"
           df -h /