chore: set environment variable using extensions in tests #6858

Job	Run time
build(21) except self-service clients	1m 11s
build(8) except for gapic-generator-java	46s
build (11)	5m 38s
build(8) for gapic-generator-java	4m 10s
build (17)	5m 38s
compatibility	1m 29s
lint	25s
showcase (11)	3m 49s
showcase-native	8m 20s
showcase (17)	4m 9s
dependency_analyzer_unit_test	18s
gapic-generator-java-bom	2m 5s
showcase-clirr	2m 57s
unmanaged_dependency_check	5m 6s
	46m 1s

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