chore: set environment variable using extensions in tests #6856

Job	Run time
build(21) except self-service clients	59s
build (11)	3m 29s
build(8) except for gapic-generator-java	57s
build (17)	3m 36s
build(8) for gapic-generator-java	44s
compatibility	41s
dependency_analyzer_unit_test	19s
gapic-generator-java-bom	46s
lint	19s
showcase (11)	40s
showcase-clirr	2m 6s
showcase (17)	41s
showcase-native	51s
unmanaged_dependency_check	2m 50s
	18m 58s

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