chore: Revert back to original source changes

Author: lqiu96

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

@@ -36,7 +36,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
 
   # Compile the Handwritten Library with the Protobuf-Java version to test source compatibility
   # Run unit tests to help check for any behavior differences (dependant on coverage)
-  mvn test -B -V -ntp \
+  mvn clean test -B -V -ntp \
       -Dclirr.skip=true \
       -Denforcer.skip=true \
       -Dmaven.javadoc.skip=true \