ci: Add env vars for split units in sdk-platform-java #1039

Job	Run time
filter	1m 0s
build(25) except self-service clients	14m 23s
build (17)	13m 52s
build(8) except for gapic-generator-java	1m 34s
build(8) for gapic-generator-java	11m 9s
build (11)	14m 18s
build(8) for showcase	9m 37s
build(21) except self-service clients	13m 45s
unmanaged_dependency_check	11m 21s
dependency_analyzer_unit_test	1m 17s
showcase-clirr	4m 50s
showcase (25)	12m 57s
gapic-generator-java-bom	2m 42s
showcase (21)	12m 26s
showcase (17)	13m 8s
showcase (11)	12m 55s
	2h 31m 14s

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