feat: bake gapic-generator-java into the hermetic build docker image #7914

Job	Run time
build (11)	14m 57s
build(21) except self-service clients	14m 2s
build (17)	15m 11s
build(8) for gapic-generator-java	11m 38s
build(8) except for gapic-generator-java	4m 11s
compatibility	1m 32s
dependency_analyzer_unit_test	16s
gapic-generator-java-bom	1m 45s
lint	21s
showcase (11)	4m 5s
showcase-clirr	3m 3s
showcase (17)	4m 1s
showcase-native	8m 11s
showcase (21)	3m 58s
unmanaged_dependency_check	4m 20s
	1h 31m 31s

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