Force Tor Browser to enable accessibility support even if no a11y feature is enabled in GNOME yet (refs: #14752, #9260).

My understanding of the code in accessible/atk/Platform.cpp is: if this envvar
is not set, Firefox will check via D-Bus at startup if accessibility is enabled
in the Desktop, and if it's not it'll permanently disable accessibility,
which breaks use cases like starting the screen keyboard or screen reader
*after* Tor Browser.