Only build debezium-core and dependences in cross-repo builds

E.g. see here: https://github.com/debezium/debezium/runs/4218021648?check_suite_focus=true

For the Vitess and Db2 connector repositories we build the entire core repository, whereas we only should build the debezium-core module and its dependences. For the Cassandra connector repository this has already been fixed via https://github.com/debezium/debezium/commit/df3f707f001b2afe7a8bd8dd4b4ff40021d01a14.