chore: bake protoc into the hermetic build docker image #6848

Job	Run time
build (11)	12m 48s
build (17)	12m 29s
build(21) except self-service clients	11m 11s
build(8) except for gapic-generator-java	4m 1s
build(8) for gapic-generator-java	9m 57s
compatibility	1m 34s
lint	21s
showcase (11)	4m 3s
showcase (17)	4m 8s
dependency_analyzer_unit_test	24s
gapic-generator-java-bom	1m 40s
showcase-clirr	2m 58s
showcase-native	8m 19s
unmanaged_dependency_check	5m 32s
	1h 19m 25s

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