[Test][Concurrency] Use env for MallocStackLogging in voucher_propagation test

Author: ramonasuncion

test/Concurrency/voucher_propagation.swift

@@ -1,7 +1,7 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-build-swift %s -target %target-swift-5.1-abi-triple -o %t/voucher_propagation
 // RUN: %target-codesign %t/voucher_propagation
-// RUN: MallocStackLogging=1 %target-run %t/voucher_propagation
+// RUN: env MallocStackLogging=1 %target-run %t/voucher_propagation
 
 // REQUIRES: executable_test
 // REQUIRES: concurrency