build: add option to force regenerate all #562

Job	Run time
filter	1m 1s
build(21) except self-service clients	12m 1s
build(8) for showcase	9m 28s
build(25) except self-service clients	12m 55s
build(8) except for gapic-generator-java	3m 52s
build(8) for gapic-generator-java	11m 4s
build (11)	14m 4s
build (17)	13m 40s
unmanaged_dependency_check	6m 50s
showcase-clirr	5m 10s
gapic-generator-java-bom	2m 22s
showcase (11)	9m 56s
showcase (25)	10m 27s
dependency_analyzer_unit_test	1m 16s
showcase (21)	9m 26s
showcase (17)	10m 27s
	2h 13m 59s

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