chore: bake the grpc plugin in the hermetic library generation image #7530

Job	Run time
build (11)	14m 44s
build(8) except for gapic-generator-java	4m 13s
build(21) except self-service clients	13m 29s
build (17)	15m 17s
build(8) for gapic-generator-java	11m 43s
compatibility	1m 27s
lint	22s
showcase (11)	5m 39s
showcase (17)	4m 7s
dependency_analyzer_unit_test	15s
gapic-generator-java-bom	1m 47s
showcase (21)	4m 14s
showcase-clirr	3m 13s
unmanaged_dependency_check	3m 56s
showcase-native	8m 26s
	1h 32m 52s

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