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...

proxmox-nixos/pkgs/nixmoxer/main.py

Line 652 in 3be878a

proxmox.upload_iso(node, "local", config["iso"] + f"/iso/{iso_file}")