package/qt6/qt6base: use DESTDIR instead of --prefix for installation

Even though it works, overriding --prefix at installation time is a
bit weird. In order to be more consistent with what is done with other
build systems, use DESTDIR instead at installation time.

Note that $(DESTDIR) comes in addition to the
-DCMAKE_INSTALL_PREFIX=/usr that is passed at configure time, so with
this commit, the files continue to be installed in $(STAGING_DIR)/usr
and $(TARGET_DIR)/usr as they should be.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/qt6/qt6base/qt6base.mk

@@ -79,11 +79,11 @@ define QT6BASE_BUILD_CMDS
 endef
 
 define QT6BASE_INSTALL_STAGING_CMDS
-	$(TARGET_MAKE_ENV) $(BR2_CMAKE) --install $(QT6BASE_BUILDDIR) --prefix $(STAGING_DIR)/usr
+	$(TARGET_MAKE_ENV) DESTDIR=$(STAGING_DIR) $(BR2_CMAKE) --install $(QT6BASE_BUILDDIR)
 endef
 
 define QT6BASE_INSTALL_TARGET_CMDS
-	$(TARGET_MAKE_ENV) $(BR2_CMAKE) --install $(QT6BASE_BUILDDIR) --prefix $(TARGET_DIR)/usr
+	$(TARGET_MAKE_ENV) DESTDIR=$(TARGET_DIR) $(BR2_CMAKE) --install $(QT6BASE_BUILDDIR)
 endef
 
 HOST_QT6BASE_DEPENDENCIES = \