chore: migrate library generation IT to cloud build #9680

Job	Run time
build (11)	14m 42s
build (17)	14m 33s
build(21) except self-service clients	13m 58s
build(8) except for gapic-generator-java	4m 7s
build(8) for gapic-generator-java	11m 10s
lint	24s
compatibility	1m 30s
gapic-generator-java-bom	1m 45s
showcase (11)	4m 29s
showcase-clirr	2m 57s
showcase (17)	4m 47s
showcase (21)	4m 32s
showcase-native	8m 52s
unmanaged_dependency_check	3m 54s
dependency_analyzer_unit_test	22s
	1h 32m 2s

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