feat: bake gapic-generator-java into the hermetic build docker image #7785

Job	Run time
build(21) except self-service clients	14m 24s
build(8) except for gapic-generator-java	4m 16s
build (11)	14m 34s
build (17)	15m 4s
build(8) for gapic-generator-java	11m 37s
compatibility	1m 34s
lint	25s
showcase (11)	3m 58s
showcase (17)	4m 1s
showcase (21)	4m 18s
dependency_analyzer_unit_test	22s
gapic-generator-java-bom	2m 12s
showcase-clirr	3m 2s
showcase-native	7m 54s
unmanaged_dependency_check	3m 50s
	1h 31m 31s

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