chore: set environment variable using extensions in tests #6867

Job	Run time
build (11)	5m 42s
build(21) except self-service clients	1m 2s
build (17)	5m 59s
build(8) except for gapic-generator-java	45s
build(8) for gapic-generator-java	3m 11s
lint	23s
compatibility	1m 26s
showcase (11)	3m 59s
showcase (17)	4m 8s
dependency_analyzer_unit_test	13s
gapic-generator-java-bom	1m 44s
showcase-clirr	3m 2s
showcase-native	8m 10s
unmanaged_dependency_check	3m 56s
	43m 40s

Provide feedback