no SVG thumbnails on testnet

[dkackman]

fix #692

change

let url = format!(
    "https://assets.mainnet.mintgarden.io/thumbnails/{data_hash}_512.webp"
);

to

let url = format!(
    "https://assets.{}.mintgarden.io/thumbnails/{data_hash}_512.webp",
    if testnet { "testnet" } else { "mainnet" }
);