bsdinstall: Fix broken netconfig persistence for installed system

The replacement of echo >> with sysrc -f used sysrc key value, which
means "read variables key and value" and thus did nothing useful (and in
fact emitted errors to the log about neither existing). Instead use the
correct sysrc key=value form so the installed system comes back up with
working networking.

Fixes: 60b37735f305 ("bsdinstall netconfig: avoid duplicate entries in rc.conf")