Implement retries for the HTTP requests

[olegbespalov]

What?

Currently, the requests for the builder service are made by using the standard HTTP client.

k6build/pkg/client/client.go

Line 107 in 6b35059

resp, err := http.DefaultClient.Do(req)

Unfortunately, the network isn't reliable and in case of its failures, it's better to have a retry mechanism in place which will re-do the HTTP request if it fails.

It will be good if it will be flexible, but maybe a simple exponential back off should be enough for the start.

Why?

Protect us from the network failures.