certctl.8: Prefer the new TRUSTDESTDIR variable

With the reimplementation in C of certctl, the environment variable
previously known as CERTDESTDIR was renamed to TRUSTDESTDIR for
consistency.

Although the previous variable is still valid, prefer the new one, as it
is described in the manual page, while the old one is not.

Reviewed by: des
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54044