ZTS: fix history_007_pos test on Ubuntu 24.04 (#16410)

The timezone "US/Mountain" isn't supported on newer linux versions.
Using the correct timezone "America/Denver" like it's done in FreeBSD
will fix this. Older Linux distros should behave also okay with this.

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>