build: skip tests in java-showcase when running bulk tests #18194

Job	Run time
gapic-bom	1m 50s
owlbot-py	1m 21s
root-pom	1m 49s
generate-readme	1m 13s
group_id_check_for_maps_libraries	8m 12s
gitignore_correctness	1m 14s
package_name_check	1m 14s
owlbot-yaml	1m 2s
	17m 55s

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