Use "build" instead of "all" when building ports modules

"all" in ports currently means "stage the ports", which requires root today,
and brings to light other potential issues, like ENAMETOOLONG with staged
directories (bug 161481, etc).

This fixes buildkernel for me when run as a non-root user, assuming all
of the prerequisites have been installed beforehand and are up-to-date.

MFC after: 1 month
Discussed with: swills (IRC)
Sponsored by: Dell EMC Isilon