Otherwise, service activation will start a new xdg-document-portal
(because we're using a private DBUS_SESSION_BUS_ADDRESS), but because
XDG_RUNTIME_DIR is shared with the real system, it won't be able to
mount its filesystem.
We need to unmount the document portal before removing the private
XDG_RUNTIME_DIR.
Signed-off-by: Simon McVittie <smcv@debian.org>