bsdinstall: Fix mirror selection

This is a follow-up to 2697622687708bffd4c3dcfc44f0c977a78e506d,
which fixed 2 out of 3 broken uses of the mirrorselect script.

Reviewed by: emaste
Approved by: emaste (src)
MFC after: 7 days
Differential Revision: https://reviews.freebsd.org/D32927

(cherry picked from commit 4042b356a00e12d6888a4e15e2373453804c5121)