build: add option to force regenerate all #498

Job	Run time
filter	58s
downstream-compatibility-spring-generator	10m 53s
downstream-compatibility (java-spanner-jdbc)	3m 19s
downstream-compatibility (java-storage)	5m 15s
downstream-compatibility (java-spanner)	5m 43s
downstream-compatibility (java-storage-nio)	3m 31s
downstream-compatibility (java-pubsub)	3m 14s
downstream-compatibility (java-pubsublite)	3m 17s
downstream-compatibility (java-firestore)	3m 30s
downstream-compatibility (java-bigtable)	7m 31s
downstream-compatibility (java-bigquery)	4m 35s
	51m 46s

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