Makefile.inc1: Fix package-pkg with a non-default LOCALBASE

package-pkg (via make-pkg-package.sh) passes CONFIGURE_ARGS to make when
building ports-mgmt/pkg, which overrides the port's default configure
args that are supposed to set --prefix. This means that pkg is always
built with the default prefix of /usr/local, which then fails when ports
tries to package it from LOCALBASE.

Work around this by explicitly adding --prefix to CONFIGURE_ARGS.

MFC after: 1 day
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D52634

(cherry picked from commit f259daa708f507d3ff450d2eb8c05065674339ec)