system/opendoas: Handle environment BUILD better.

Signed-off-by: B. Watson <yalhcru@gmail.com>

system/opendoas/opendoas.SlackBuild

@@ -100,10 +100,7 @@ fi
 
 # Passing BUILD from the command line otherwise causes the build to fail (the
 # configure script uses the variable name).
-SBBUILD=$BUILD
-unset BUILD
-
-./configure \
+env -u BUILD ./configure \
   --prefix=/usr \
   $SHADOW \
   $TIMESTAMP \
@@ -117,8 +114,6 @@ make install DESTDIR=$PKG
 strip $PKG/usr/bin/*
 gzip -9 $PKG/usr/man/man*/*
 
-BUILD=$SBBUILD
-
 mkdir -p $PKG/usr/doc/$PRGNAM-$VERSION
 cp -a \
   LICENSE README.md \