sysutils/vagrant: Respect hier(7) for networks.conf

Since virtualbox-ose 6.1.26 there is a new networks.conf file
which had originally been placed in /etc/vbox.

In 66635b2061daf this was been corrected for emulators/virtualbox-ose,
this change does the same for vagrant's virtualbox plugin.

Before this change, networks.conf had to be put into two places
(e.g., by symlinking /usr/local/etc/vbox to /etc/vbox).

PR: 266845
Approved by: joe@thrallingpenguin.com (maintainer)