Add an example to downgrade a package

lib/mk_computer.nix

@@ -15,6 +15,17 @@ let
     overlays = [
       overlay-unstable
       inputs.nix-alien.overlay
+      #      (final: prev: {
+      #        nextcloud-client = prev.nextcloud-client.overrideAttrs (_: rec {
+      #          version = "3.6.0";
+      #          src = pkgs.fetchFromGitHub {
+      #            owner = "nextcloud";
+      #            repo = "desktop";
+      #            rev = "v${version}";
+      #            sha256 = "sha256-wAxq5xFlofn2xEguvewMvGcel9O+CN/1AycR3tv/xMA=";
+      #          };
+      #        });
+      #      })
     ];
   };
 in