ci: Allow manual invocations for the Protobuf Source CI Job (#3463) #87

Job	Run time
build (11)	2m 54s
build (17)	2m 49s
build(8) except for gapic-generator-java	2m 49s
build(21) except self-service clients	2m 49s
build(8) for gapic-generator-java	2m 49s
compatibility	1m 45s
dependency_analyzer_unit_test	24s
gapic-generator-java-bom	2m 5s
lint	32s
showcase (11)	2m 48s
showcase-clirr	0s
showcase (17)	2m 47s
showcase-native	2m 47s
showcase (21)	2m 34s
unmanaged_dependency_check	7m 30s
	37m 22s

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