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

Makefile

@@ -195,11 +195,11 @@ PATH=	/sbin:/bin:/usr/sbin:/usr/bin
 MAKEOBJDIRPREFIX?=	/usr/obj
 _MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} MK_AUTO_OBJ=no \
     ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} __MAKE_CONF=${__MAKE_CONF} \
-    SRCCONF=${SRCCONF} \
+    SRCCONF=${SRCCONF} SRC_ENV_CONF= \
     -f /dev/null -V MAKEOBJDIRPREFIX dummy
 .if !empty(_MAKEOBJDIRPREFIX)
-.error MAKEOBJDIRPREFIX can only be set in environment, not as a global\
-	(in make.conf(5) or src.conf(5)) or command-line variable.
+.error MAKEOBJDIRPREFIX can only be set in environment or src-env.conf(5),\
+    not as a global (in make.conf(5) or src.conf(5)) or command-line variable.
 .endif
 
 # We often need to use the tree's version of make to build it.