nixos/sway-beta: Improve the wrapper

According to the dbus-launch documentation [0] "--exit-with-session"
shouldn't be used: "This option is not recommended, since it will
consume input from the terminal where it was started; it is mainly
provided for backwards compatibility." And it also states: "To start a
D-Bus session within a text-mode session, do not use dbus-launch.
Instead, see dbus-run-session(1)."

The new wrapper also avoids starting an additional D-Bus session if
DBUS_SESSION_BUS_ADDRESS is already set.

Fix #51303.

[0]: https://dbus.freedesktop.org/doc/dbus-launch.1.html
[1]: https://dbus.freedesktop.org/doc/dbus-run-session.1.html