ovmf-prebuilt: Add download retry

[nicholasbishop]

If the download fails, retry up to four times. The first retry will happen after a one-second delay. Each subsequent retry doubles the delay.

Fixes #244

[mkroening]

Thanks! This should help with spurious 504 Gateway Timeouts in GitHub-hosted runners.

I noticed two things:

1. This retries on all errors, not just transient errors.

   Transient error means either: a timeout, an FTP 4xx response code or an HTTP 408, 429, 500, 502, 503, 504, 522 or 524 response code.

   From Curl --retry

   Non-transient errors, such as 404 Not Found, don't benefit from being retried. Retrying non-transient errors only wastes a bit of time, though, and differentiating may not be worth the effort.

2. This does not respect the HTTP Retry-After response header.

   Supporting that header is probably also not worth the effort, though.

The reqwest::retry could help with those goals, but that would also take longer to compile.

So this PR looks good to me and will solve the spurious CI failures. Thanks again! :)