OSX CI builds fail to install python

[akoeplinger]

https://dev.azure.com/dnceng/public/_build/results?buildId=555403&view=results
https://dev.azure.com/dnceng/public/_build/results?buildId=555050&view=results

Error: python 3.7.6_1 is already installed
To upgrade to 3.7.7, run `brew upgrade python`.

Seems to affect all OSX builds.

[directhex]

Do we want to add a brew upgrade call prior to the brew install calls, so any packages already installed get upgraded to $latest (calling install on a package which is already $latest is a warn, not an error)?