chore(hermetic-build): explore parallel generation #1070

Job	Run time
filter	1m 6s
downstream-compatibility-spring-generator	11m 42s
downstream-compatibility (java-bigtable)	5m 50s
downstream-compatibility (java-pubsub)	3m 45s
downstream-compatibility (java-firestore)	3m 49s
downstream-compatibility (java-pubsublite)	4m 59s
	31m 11s

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