Nixmoxer: Make iso storage location configurable

[max06]

My proxmox installations are running of a small 16G boot-disk/local storage with additional storage options available.

It would be great if the temporary storage location for the nixox installation image could be modified.

I currently can't use it because the generated image is 3.5G in size, 500mb too much...

https://github.com/SaumonNet/proxmox-nixos/blob/3be878a84866b9ef9214b8ea6f53630f47f4b192/pkgs/nixmoxer/main.py#L652