chore: migrate library generation IT to cloud build #9626

Job	Run time
build (11)	14m 59s
build (17)	14m 32s
build(21) except self-service clients	14m 14s
build(8) except for gapic-generator-java	4m 21s
build(8) for gapic-generator-java	11m 8s
compatibility	1m 33s
lint	32s
showcase (11)	4m 23s
showcase (17)	4m 30s
showcase (21)	4m 36s
dependency_analyzer_unit_test	20s
gapic-generator-java-bom	1m 58s
showcase-clirr	3m 7s
showcase-native	8m 35s
unmanaged_dependency_check	3m 53s
	1h 32m 41s

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