Allow setting `fileIOConcurrency` using environment variables

[fourdim]

It's not possible to pass fileIOConcurrency using parameters when building projects on vercel as far as I know. Is it possible to have an environment variable like NFT_FILE_IO_CONCURRENCY to control this behaviour?

[styfle]

The original PR didn't have much detail but I assume the number was chosen to suit Vercel's default build container.

• https://github.com/vercel/nft/pull/301

I don't think we would want users to change it and cause OOM errors.

Is there a problem you're trying to solve?

[fourdim]

Yes, thanks.