chore: set environment variable using extensions in tests #6859

Job	Run time
build (11)	5m 7s
build(8) except for gapic-generator-java	48s
build (17)	5m 24s
build(8) for gapic-generator-java	3m 8s
build(21) except self-service clients	1m 0s
compatibility	1m 35s
dependency_analyzer_unit_test	19s
gapic-generator-java-bom	1m 46s
lint	33s
showcase (11)	4m 8s
showcase-clirr	3m 3s
showcase (17)	4m 7s
showcase-native	8m 1s
unmanaged_dependency_check	4m 58s
	43m 57s

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