cve-update-nvd2-native: use exact times, don't truncate

When requesting updates in a specific range, use the actual current time
and database mtime instead of truncating to midnight, and explicitly set
the timezone to UTC so that NIST don't treat the timestamps as _their_ local
time when they're _our_ local time.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>