ci: Allow manual invocations for the Protobuf Source CI Job (#3463) #9567

Job	Run time
build(8) except for gapic-generator-java	4m 16s
build (11)	14m 33s
build(21) except self-service clients	13m 13s
build (17)	14m 36s
build(8) for gapic-generator-java	11m 10s
compatibility	1m 31s
gapic-generator-java-bom	1m 39s
lint	22s
showcase (11)	4m 26s
showcase-native	8m 27s
showcase (17)	4m 46s
showcase (21)	4m 7s
dependency_analyzer_unit_test	11s
showcase-clirr	0s
unmanaged_dependency_check	3m 58s
	1h 27m 15s

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