ci: Allow manual invocations for the Protobuf Source CI Job #5160

Job	Run time
downstream-compatibility-spring-generator	8m 49s
downstream-compatibility (google-cloud-java)	11m 21s
downstream-compatibility (java-bigtable)	3m 41s
downstream-compatibility (java-bigquery)	2m 55s
downstream-compatibility (java-bigquerystorage)	9m 29s
downstream-compatibility (java-datastore)	2m 41s
downstream-compatibility (java-firestore)	1m 52s
downstream-compatibility (java-logging)	2m 17s
downstream-compatibility (java-logging-logback)	1m 27s
downstream-compatibility (java-pubsub)	2m 15s
downstream-compatibility (java-pubsublite)	2m 12s
downstream-compatibility (java-spanner-jdbc)	2m 13s
downstream-compatibility (java-spanner)	4m 16s
downstream-compatibility (java-storage)	4m 13s
downstream-compatibility (java-storage-nio)	2m 11s
	1h 1m 52s

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