stat: Set the timezone before testing -t flag

The test assumes UTC, which is what I use on my development systems and
clearly what is used on our CI runners.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D56836

usr.bin/stat/tests/stat_test.sh

@@ -1,7 +1,7 @@
 #
 # Copyright (c) 2017 Dell EMC
 # All rights reserved.
-# Copyright (c) 2025 Klara, Inc.
+# Copyright (c) 2025-2026 Klara, Inc.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions
@@ -234,9 +234,9 @@ t_flag_head()
 {
 	atf_set	"descr" "Verify the output format for -t"
 }
-
 t_flag_body()
 {
+	export TZ=UTC
 	atf_check touch foo
 	atf_check touch -d 1970-01-01T00:00:42 foo
 	atf_check -o inline:'42\n' \