ci: Allow manual invocations for the Protobuf Source CI Job #9565

Job	Run time
build(21) except self-service clients	13m 37s
build (11)	14m 39s
build (17)	14m 52s
build(8) except for gapic-generator-java	4m 22s
build(8) for gapic-generator-java	11m 10s
compatibility	1m 49s
lint	25s
dependency_analyzer_unit_test	12s
gapic-generator-java-bom	1m 53s
showcase (11)	4m 32s
showcase (17)	4m 25s
showcase (21)	4m 25s
showcase-clirr	3m 1s
showcase-native	8m 42s
unmanaged_dependency_check	5m 35s
	1h 33m 39s

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