chore: migrate library generation IT to cloud build #9588

Job	Run time
build (11)	14m 38s
build(21) except self-service clients	13m 26s
build(8) except for gapic-generator-java	4m 31s
build (17)	14m 17s
build(8) for gapic-generator-java	11m 22s
compatibility	1m 34s
lint	21s
dependency_analyzer_unit_test	11s
gapic-generator-java-bom	1m 52s
showcase (11)	4m 38s
showcase-clirr	3m 5s
showcase (17)	4m 20s
showcase (21)	4m 20s
showcase-native	8m 58s
unmanaged_dependency_check	5m 31s
	1h 33m 4s

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