chore(hermetic-build): explore parallel generation #1252

Job	Run time
filter	1m 10s
build(8) for showcase	9m 25s
build(8) except for gapic-generator-java	3m 23s
build(8) for gapic-generator-java	11m 7s
unmanaged_dependency_check	10m 41s
showcase (25)	14m 35s
gapic-generator-java-bom	2m 35s
dependency_analyzer_unit_test	1m 22s
showcase (11)	15m 11s
showcase (21)	13m 55s
build (17)	13m 59s
build (11)	14m 35s
showcase (17)	12m 57s
showcase-clirr	5m 3s
build(21) except self-service clients	13m 41s
build(25) except self-service clients	13m 37s
	2h 37m 16s

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