ci(self-hosted): clean before run

.github/workflows/build_slim_packages.yaml

@@ -43,6 +43,7 @@ jobs:
     container: "ghcr.io/emqx/emqx-builder/5.0-10:${{ matrix.elixir }}-${{ matrix.otp }}-${{ matrix.os }}"
 
     steps:
+    - uses: AutoModality/action-clean@v1
     - uses: actions/checkout@v1
     - name: prepare
       run: |