chore: update instructions on `generate_repo.py` #5148

Job	Run time
build(21) except self-service clients	11m 12s
build(8) except for gapic-generator-java	3m 36s
build (11)	11m 46s
build(8) for gapic-generator-java	9m 15s
build (17)	11m 50s
lint	20s
compatibility	1m 49s
gapic-generator-java-bom	1m 47s
showcase (11)	4m 53s
showcase-clirr	3m 14s
showcase (17)	4m 51s
unmanaged_dependency_check	4m 22s
showcase-native	9m 28s
	1h 18m 23s

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