Follow-up to bug 1065180: Replace a use of environment with os.getenv in a test I somehow missed earlier. no_r=me, testonly.

--HG--
extra : rebase_source : c72c52715c16b8476ebb642dd7b8c7a71c231003
extra : amend_source : 5b280326bfd1f047bc936ad6074f22e62e32cc45