[libc++] Use env to set the tool's environment on Darwin

Author: ldionne

libcxx/utils/libcxx/test/config.py

@@ -1043,7 +1043,7 @@ def configure_substitutions(self):
         if self.target_info.is_darwin():
             # Do not pass DYLD_LIBRARY_PATH to the compiler, linker, etc. as
             # these tools are not meant to exercise the just-built libraries.
-            tool_env += 'DYLD_LIBRARY_PATH="" '
+            tool_env += 'env DYLD_LIBRARY_PATH="" '
 
         sub = self.config.substitutions
         cxx_path = tool_env + pipes.quote(self.cxx.path)