SchemeWithRetries.Fetch improved timeout handling

[HouseK]

Is your feature request related to a problem? Please describe.
Exponential backoff takes longer to retry than expected.

Describe the solution you'd like

u-root/pkg/curl/schemes.go

Line 330 in 3ad1ae2

func (s *SchemeWithRetries) Fetch(ctx context.Context, u *url.URL) (io.ReaderAt, error) {

Don't sleep for the full NextBackOff duration, but only for the remainder after subtracting the time Fetch took.

Pass a context to Fetch with a timeout equal to the nextbackoff duration.

Describe alternatives you've considered
Doing nothing. This still works but is IMO just a bit less accurate since long timeouts actually slow down the retries.