[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).

Makefile

@@ -149,7 +149,7 @@ build_gui: build_golib relink_vendor
 	@echo "==============BUILD GUI==============="
 	@echo "TARGET: ${TARGET}"
 	@echo "VENDOR_PATH: ${VENDOR_PATH}"
-	@XBUILD=no CC=${CC} CXX=${CXX} MAKE=${MAKE} AR=${AR} LD=${LD} QMAKE=${QMAKE} LRELEASE=${LRELEASE} TARGET=${TARGET} VENDOR_PATH=${VENDOR_PATH} APPNAME=${APPNAME} gui/build.sh --skip-golib
+	@XBUILD=no CC=${CC} CXX=${CXX} MAKE=${MAKE} AR=${AR} LD=${LD} LRELEASE=${LRELEASE} TARGET=${TARGET} VENDOR_PATH=${VENDOR_PATH} APPNAME=${APPNAME} gui/build.sh --skip-golib
 	@echo "============BUILD GUI================="
 
 build: build_helper build_gui