chore: migrate library generation IT to cloud build #9582

Job	Run time
build (11)	14m 25s
build (17)	14m 44s
build(21) except self-service clients	13m 40s
build(8) for gapic-generator-java	11m 8s
build(8) except for gapic-generator-java	4m 16s
compatibility	1m 47s
gapic-generator-java-bom	1m 57s
lint	21s
showcase (11)	4m 17s
showcase (17)	4m 25s
showcase-clirr	3m 4s
showcase-native	8m 46s
showcase (21)	4m 20s
unmanaged_dependency_check	4m 11s
dependency_analyzer_unit_test	18s
	1h 31m 39s

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