chore: Fix CI issue

Author: lqiu96

.kokoro/presubmit/downstream-protobuf-source-compatibility.sh

@@ -27,7 +27,10 @@ if [ -z "${PROTOBUF_RUNTIME_VERSION}" ]; then
   exit 1
 fi
 
+# Get the directory of the build script
+scriptDir=$(realpath "$(dirname "${BASH_SOURCE[0]}")")
 cd "${scriptDir}/../.." # cd to the root of this repo
+source "$scriptDir/common.sh"
 
 setup_maven_mirror