Add support for an env_str in mobly #928

Job	Run time
build-and-test (macos-latest, 3.12)	29s
build-and-test (ubuntu-latest, 3.11)	22s
build-and-test (macos-latest, 3.11)	26s
build-and-test (windows-latest, 3.11)	36s
build-and-test (windows-latest, 3.12)	29s
build-and-test (ubuntu-latest, 3.12)	26s
	2m 48s

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