cask/upgrade: don't uninstall before upgrade

[bevanjkay]

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This is an initial pass at trying to skip the uninstall step when running brew upgrade and brew reinstall.
I have also added a simple dsl uninstall_on_upgrade! that would cause the uninstall to always run, for use in cases where it may be required.

This is currently only accessible behind an ENV variable - HOMEBREW_NO_UNINSTALL_ON_CASK_UPGRADE=1.

I have tested this with simple applications so far and it seems to work ok, but have not completed any testing with pkg or script installers yet.

One blindside we may have here is that we do not test brew upgrade on CI, which means any future issues would only be surfaced by users, unless we had a way of running the upgrade on PRs.

[bevanjkay]

We will likely need a way to detect changes in artifacts between the old and new versions, and where there are changes remove the old. Otherwise we will end up with multiple versions of applications when paths change, or names change etc...

[bevanjkay]

I wonder if we do a straight comparison to see if any artifacts have changed, and if they have, we then go ahead with the uninstall as we historically have to ensure we clean up correctly?