support: Pass environ to child process

Pass environ to posix_spawn so that the child process can inherit

environment of the test.

(cherry picked from commit e958490f8c74e660bd93c128b3bea746e268f3f6)

diff --git a/support/support_subprocess.c b/support/support_subprocess.c

index 12c79ff6b0859877..4573350d775ac4c8 100644

--- a/support/support_subprocess.c

+++ b/support/support_subprocess.c

@@ -84,7 +84,7 @@ support_subprogram (const char *file, char *const argv[])

   xposix_spawn_file_actions_addclose (&fa, result.stdout_pipe[1]);

   xposix_spawn_file_actions_addclose (&fa, result.stderr_pipe[1]);

-  result.pid = xposix_spawn (file, &fa, NULL, argv, NULL);

+  result.pid = xposix_spawn (file, &fa, NULL, argv, environ);

   xclose (result.stdout_pipe[1]);

   xclose (result.stderr_pipe[1]);