ci: use nested directories

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -43,18 +43,16 @@ jobs:
         # which values to use and would resolve to ''.
         protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_runtime_versions || '"3.25.5","4.28.3"')) }}
     steps:
+      - uses: actions/checkout@v4
       - uses: actions/checkout@v4
         with:
-          path: sdk-platform-java
+          repository: "googleapis/${{ matrix.repo }}"
+          path: "${{ matrix.repo }}"
       - uses: actions/setup-java@v4
         with:
           # Use Java 11 for this as Linkage Checker is only compatible with Java 11 or below
           java-version: 11
           distribution: temurin
-      - uses: actions/checkout@v4
-        with:
-          repository: "googleapis/${{ matrix.repo }}"
-          path: "${{ matrix.repo }}"
       - run: |
           pwd
           pushd "${{ matrix.repo }}"