Fix the '-a' option to ${PORTSDIR}/Tools/scripts/addport

Summary:
Currently addport -a ends up running a command line like:

make  PORTSDIR="/tmp/ap.BsOuZJBy" clean check-categories

where the temporary directory has only parts of the ports tree
(specifically bits under ${PORTSDIR}/Mk) checked out. This can't work
as 'make check-categories' needs to see the entire ports tree checked
out.

Instead, run the checks specified by '-a' and '-t' with the value of
$PORTSDIR from the environment.

Reviewers: crees

Reviewed By: crees

Subscribers: mat

Differential Revision: https://reviews.freebsd.org/D7525