[Makefile] Don't overwrite QMAKE env when calling build.sh

gui/build.sh already knows how to find the right qmake executable. We
don't have to overwrite it here. On ArchLinux, we need qmake6 instead of
qmake. The Makefile sets QMAKE=qmake, even if `gui/build.sh` knows
better (but not if QMAKE is already set).