Add QT_VIRTUALKEYBOARD_STYLE_PATH env var

While there is an environment varialbe for setting the keyboard layout (QT_VIRTUALKEYBOARD_LAYOUT_PATH), there is no environment variable to set the style. This must be done by setting QML_IMPORT_PATH to a directory that contains QtQuick/VirtualKeyboard/Styles subdirectories. This is quite cumbersome. Add a dedicated environment variable evaluation for style as well to improve consistency to qquickvirtualkeyboardsettings.cpp stylePath() method.