Set OPTIONS_UNSET in the argument list to env(1), and add
AVAHI to the list. This fixes the textproc/docproj build
seemingly following FLAVORS being added.

Specifically, the problem with the dependency chain here is:

• textproc/docproj depends on print/cups, which sets AVAHI=on by default;
• net/avahi-app depends on devel/gobject-introspection, which requires python3+;
• graphics/netpbm depends on graphics/mesa-libs, which can only be built with python2.7;
• textproc/docproj depends on a number of graphics ports for font rendering, etc.

MFC after: 3 days
MFC before: 12.0-BETA3
Sponsored by: The FreeBSD Foundation