fetch: add --retry option to retry fetch once.

Sometimes there may be intermittent failures with hosts that work if
immediately retried. Let's allow a single retry in this case with the
--retry flag. This also behaves nicely with the --force flag.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
brew-test-bot: use fetch --retry.

Closes #27110.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>