mythburn.py: pass the starttime in utc format to mythtranscode

I can't easily test this but hopefully this will fix the problems reported on
the user mailing list and in #11758.

Refs #11758.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>
(cherry picked from commit 45e62c8cb69c3abbbf883d926bd5c0e2137d7365)