nixos/tests/gitea: increase memory for VM

Otherwise, the relevant processes are stopped by the kernel's OOM
killer[1].

[1] https://github.com/NixOS/nixpkgs/pull/105698#issuecomment-737257293

nixos/tests/gitea.nix

@@ -14,6 +14,7 @@ let
 
     nodes = {
       server = { config, pkgs, ... }: {
+        virtualisation.memorySize = 2048;
         services.gitea = {
           enable = true;
           database = { inherit type; };