tests/c_lib: Ensure TZ value is UTC for ctime/ctime_r tests

When testing these functions with CONFIG_NATIVE_LIBC, force the TZ
environment variable to UTC to ensure they produce the expected results.

Signed-off-by: Keith Packard <keithp@keithp.com>

tests/lib/c_lib/common/src/main.c

@@ -1166,6 +1166,9 @@ ZTEST(libc_common, test_time_ctime)
 	char buf[26] = {0};
 	time_t test1 = 1718260000;
 
+#ifdef CONFIG_NATIVE_LIBC
+	setenv("TZ", "UTC", 1);
+#endif
 	zassert_not_null(ctime_r(&test1, buf));
 	zassert_equal(strncmp("Thu Jun 13 06:26:40 2024\n", buf, sizeof(buf)), 0);