Use retry script to publish codecov reports on Windows

[nicoddemus]

Currently it is common for entire AppVeyor builds (which already take a lot of time) to fail because some intermittent failure to upload the codecov report.

Found this script which we might use to retry the command a few times before effectively giving up:

https://github.com/haskell/cabal/blob/master/appveyor-retry.cmd

Follow up from: #4383 (comment)

[blueyed]

Hmm, maybe we can use codecov-bash there also? (which has re-trying and is meant to become the go-to implementation)
codecov/codecov-bash#56