Disable file cache for shadergen

QtQuick3D's shadergen uses cache from user's home directory.
Disable the caching as it can lead to problems with pseudo.

Change-Id: I6b889fe94eb218d803ece49c9e0ca4ec18c3524a
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>

classes/qt6-cmake.bbclass

@@ -42,3 +42,5 @@ do_install:append() {
                -e 's|${STAGING_DIR_HOST}|$$[QT_SYSROOT]|g' \
                -e '/QMAKE_PRL_BUILD_DIR/d' {} \;
 }
+
+export QT_DISABLE_SHADER_DISK_CACHE = "1"