build: add option to force regenerate all #18635

Job	Run time
windows	2m 12s
enforcer	18m 46s
changes	5s
generation-config-check	1m 41s
gapic-libraries-bom	18m 16s
units (11)	43m 18s
lint	4m 18s
units (21)	34m 23s
units (17)	35m 20s
units (8)	41m 31s
units (25)	33m 10s
split-units (sdk-platform-java, 8)	4m 16s
split-units (sdk-platform-java, 21)	3m 59s
split-units (sdk-platform-java, 17)	4m 50s
split-units (sdk-platform-java, 25)	4m 54s
split-units (sdk-platform-java, 11)	4m 23s
	4h 15m 22s

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