qtbase: remove QT_SOURCE_TREE setting

Other module builds will try to use QT_SOURCE_TREE which points to the
qtbase build area. Builds can then break if qtbase build area is
cleaned.

recipes-qt/qt6/qtbase_git.bb

@@ -115,6 +115,9 @@ EXTRA_OECMAKE_append_class-target = "\
 SYSROOT_DIRS += "${prefix}/mkspecs"
 
 do_install_append() {
+    sed -i ${D}${libdir}/cmake/Qt6BuildInternals/QtBuildInternalsExtra.cmake \
+        -e '/QT_SOURCE_TREE/,+2d'
+
     # qt-cmake is not needed, remove it and the toolchain file
     rm -f ${D}${QT6_INSTALL_BINDIR}/qt-cmake \
           ${D}{QT6_INSTALL_LIBDIR}/cmake/Qt6/qt.toolchain.cmake