ci: parallelize downstream and showcase native testing #6623

Job	Run time
build (11)	13m 0s
build (17)	12m 11s
build(21) except self-service clients	11m 25s
build(8) except for gapic-generator-java	4m 0s
build(8) for gapic-generator-java	9m 21s
compatibility	1m 50s
dependency_analyzer_unit_test	24s
gapic-generator-java-bom	2m 5s
showcase (11)	3m 52s
showcase (17)	3m 55s
lint	25s
showcase-clirr	2m 56s
showcase-native	8m 19s
unmanaged_dependency_check	3m 46s
	1h 17m 29s

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