ci: test downstream CI #5450

Job	Run time
build (11)	11m 54s
build (17)	12m 49s
build(21) except self-service clients	11m 21s
build(8) except for gapic-generator-java	3m 45s
build(8) for gapic-generator-java	9m 45s
compatibility	2m 27s
lint	41s
showcase (11)	3m 54s
showcase-clirr	3m 15s
showcase (17)	3m 50s
showcase-native	8m 23s
gapic-generator-java-bom	4m 16s
unmanaged_dependency_check	3m 37s
	1h 19m 57s

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