chore: set environment variable using extensions in tests #6870

Job	Run time
build(21) except self-service clients	10m 53s
lint	29s
build(8) except for gapic-generator-java	50s
build (11)	12m 10s
build (17)	11m 51s
build(8) for gapic-generator-java	9m 17s
compatibility	1m 33s
gapic-generator-java-bom	1m 57s
showcase (11)	4m 8s
showcase-clirr	3m 5s
showcase (17)	4m 28s
showcase-native	8m 19s
dependency_analyzer_unit_test	13s
unmanaged_dependency_check	4m 48s
	1h 14m 1s

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