build: add option to force regenerate all #567

Job	Run time
filter	1m 5s
build(8) except for gapic-generator-java	3m 56s
build(21) except self-service clients	12m 54s
build(8) for gapic-generator-java	11m 27s
build(25) except self-service clients	12m 33s
dependency_analyzer_unit_test	1m 24s
showcase-clirr	5m 26s
build (17)	14m 12s
gapic-generator-java-bom	2m 17s
build(8) for showcase	9m 33s
build (11)	13m 51s
showcase (21)	10m 6s
showcase (25)	10m 21s
showcase (17)	9m 53s
unmanaged_dependency_check	6m 52s
showcase (11)	9m 57s
	2h 15m 47s

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