chore: migrate library generation IT to cloud build #9667

Job	Run time
build(8) except for gapic-generator-java	4m 14s
build(8) for gapic-generator-java	11m 8s
build (11)	14m 47s
build(21) except self-service clients	13m 15s
compatibility	1m 35s
build (17)	14m 58s
gapic-generator-java-bom	1m 51s
lint	22s
showcase (11)	4m 29s
showcase-clirr	3m 13s
showcase (17)	4m 19s
showcase-native	8m 33s
showcase (21)	4m 20s
dependency_analyzer_unit_test	23s
unmanaged_dependency_check	5m 35s
	1h 33m 2s

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