fix: error on vm-bigdisk flake configs

[bogdan-manole]

A fix for the bellow error when I run nix develop --command nixos-shell --flake .#vm-bigdisk

error: value is a set while an integer was expected

       at /nix/store/w3v5jwygrqwifrj1s85gngikivdpq7yf-source/flake.nix:175:53:

          174|             nixos-shell.nixosModules.nixos-shell
          175|             ({lib, ...}: {virtualisation.diskSize = lib.mkForce 1024 * 60;})
             |                                                     ^
          176|           ];

[dermetfan]

Thanks!