pull: do fetch after everything else, sleep more.
pull: do fetch after everything else, sleep more.

Use an exponential back-off on the sleep and try the fetch more times.

Hopefully this will mean that we spent less time waiting for Bintray.

Closes Homebrew/homebrew#43240.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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