[xpadneo] Moved to it's own config. Adds an env var for steam.

system/hosts/altair/default.nix

@@ -11,6 +11,7 @@
     ../../features/gc.nix
     ../../features/gui.nix
     ../../features/virtualbox.nix
+    ../../features/xpadneo.nix
     ./configuration.nix
     ./hardware-configuration.nix
     ./qmk.nix