ci: do not generate ubuntu jobs if not specified
ci: do not generate ubuntu jobs if not specified

We used to generate ubuntu only jobs even if ubuntu is not in the os
list, wrap them with os checkers now.

greenplum-db/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb