docs: modify hermetic build docs #8899

Job	Run time
build(8) except for gapic-generator-java	4m 30s
build(21) except self-service clients	14m 0s
build (11)	15m 1s
build (17)	15m 41s
lint	27s
build(8) for gapic-generator-java	12m 2s
compatibility	1m 49s
dependency_analyzer_unit_test	27s
gapic-generator-java-bom	2m 5s
showcase (11)	4m 34s
showcase (17)	4m 13s
showcase-clirr	2m 58s
showcase (21)	4m 15s
showcase-native	8m 9s
unmanaged_dependency_check	5m 9s
	1h 35m 20s

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