nixos/desktop-managers/xterm: Defaults to xserver's state

This enhances #61423, which removed the gating of desktop-managers from
being linked to the xserver's state.

This, though, brought in xterm into all systems, even those without X
servers.

This change sets the *default* of the xterm desktop-manager to the state
of the xserver, keeping it enabled by default as a sane fallback.

The xterm desktop-manager can still be enabled or disabled as needed,
without it being affected by xserver's state.