chore: add junit5 extension to setup environment variable #6873

Job	Run time
build (11)	12m 4s
build (17)	12m 9s
build(21) except self-service clients	10m 42s
build(8) except for gapic-generator-java	59s
build(8) for gapic-generator-java	9m 22s
compatibility	1m 32s
dependency_analyzer_unit_test	28s
gapic-generator-java-bom	2m 23s
lint	34s
showcase (11)	4m 8s
showcase (17)	4m 35s
showcase-clirr	2m 59s
showcase-native	8m 13s
unmanaged_dependency_check	5m 31s
	1h 15m 39s

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