chore: migrate library generation IT to cloud build #9618

Job	Run time
build(21) except self-service clients	13m 27s
build (11)	15m 1s
build(8) except for gapic-generator-java	5m 1s
build (17)	14m 39s
build(8) for gapic-generator-java	11m 31s
compatibility	1m 55s
lint	29s
showcase (11)	4m 42s
showcase (17)	4m 48s
showcase (21)	4m 19s
dependency_analyzer_unit_test	38s
gapic-generator-java-bom	2m 6s
showcase-clirr	3m 25s
showcase-native	8m 43s
unmanaged_dependency_check	4m 23s
	1h 35m 7s

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