build: add option to force regenerate all #587

Job	Run time
filter	1m 2s
build(25) except self-service clients	12m 5s
build (17)	13m 27s
build (11)	13m 37s
build(8) for gapic-generator-java	11m 33s
build(21) except self-service clients	12m 15s
build(8) for showcase	9m 37s
unmanaged_dependency_check	10m 43s
gapic-generator-java-bom	2m 20s
showcase-clirr	6m 1s
dependency_analyzer_unit_test	1m 22s
build(8) except for gapic-generator-java	3m 52s
showcase (25)	11m 22s
showcase (11)	10m 49s
showcase (17)	10m 58s
showcase (21)	10m 38s
	2h 21m 41s

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