pkgbase: Fix OSVERSION specification when creating a repo

-o OSVERSION= needs to appear before the "repo" verb, otherwise it has
no effect. In this case, recent pkg-devel fails to create the repo,
saying that ABI cannot be specified without OSVERSION.

Reviewed by: kevans, manu
MFC after: 2 weeks
Fixes: 188fe88ec50e ("pkgbase: force OSVERSION")
Differential Revision: https://reviews.freebsd.org/D48518