Don't read SRC_ENV_CONF for MAKEOBJDIRPREFIX guard.

This is mostly to allow using MAKEOBJDIRPREFIX in src-env.conf on stable where
src.sys.obj.mk is not going to be MFC'd. It is still valid on head but
effectively a NOP due to MAKEOBJDIRPREFIX being handled differently in
src.sys.obj.mk.

Reported by: eadler
MFC after: 1 week
Sponsored by: Dell EMC