chore: migrate library generation IT to cloud build #9627

Job	Run time
build (11)	14m 41s
build (17)	14m 18s
build(21) except self-service clients	13m 48s
build(8) except for gapic-generator-java	4m 25s
build(8) for gapic-generator-java	11m 39s
compatibility	1m 34s
lint	26s
dependency_analyzer_unit_test	21s
gapic-generator-java-bom	1m 53s
showcase (11)	4m 42s
showcase (17)	4m 36s
showcase-clirr	3m 13s
showcase (21)	4m 34s
showcase-native	8m 42s
unmanaged_dependency_check	4m 3s
	1h 32m 55s

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