steam: Move to system instead of Flatpak

- Support of gamescope for HDR
- Best integration with udev devices
- Add proton-ge

hosts/perso-desktop/configuration.nix

@@ -20,6 +20,10 @@
   networking.hostName = "nixos-desktop-perso"; # Define your hostname.
 
   modules.system = {
+    apps = {
+      steam.enable = true;
+    };
+
     desktop = {
       plasma.enable = true;
     };