chore: add junit5 extension to setup environment variable #6881

Job	Run time
build(21) except self-service clients	10m 58s
build(8) except for gapic-generator-java	1m 41s
build (11)	12m 3s
build (17)	11m 56s
compatibility	1m 43s
build(8) for gapic-generator-java	9m 22s
lint	23s
showcase (11)	3m 52s
showcase (17)	4m 14s
dependency_analyzer_unit_test	15s
gapic-generator-java-bom	1m 49s
showcase-clirr	3m 10s
showcase-native	7m 59s
unmanaged_dependency_check	4m 44s
	1h 14m 9s

Provide feedback