Unconditionally upgrade installed Brew packages on OSX (#33481)
With Apt, if you have X 1.0 installed, 1.1 exists in the package manifest, and say "install X", X is upgraded to 1.1. With Brew, a fatal error is thrown.
Blindly upgrading all previously installed packages is not ideal, and not a perfect mirror for the Apt behaviour, but it should mean that if X is previously installed it gets upgraded (causing a warning during install, when it's asked to be installed again with the same version), and if it's not already installed then it gets installed later.
Closes: https://github.com/dotnet/runtime/issues/33471