neofetch: default x11Support to true on Linux only

neofetch currently fails to build on platforms other than Linux with
x11Support = true as ueberzug 18.3.1 only works on Linux (see #437381).
Only set x11Support to true for Linux. Other platforms could be
supported by switching to ueberzugpp.

Fixes: #436876

pkgs/by-name/ne/neofetch/package.nix

···
   bash,
   makeWrapper,
   pciutils,
-  x11Support ? !stdenvNoCC.hostPlatform.isOpenBSD,
+  x11Support ? stdenvNoCC.hostPlatform.isLinux,
   ueberzug,
   fetchpatch,
 }: