cask/installer: remove migration symlinks on uninstall

[EricFromCanada]

Fixes #19951 by ensuring @cask.tap is set during uninstall, allowing @cask.old_tokens to be populated. Thanks to @tyuwags for the hints. Replaces #20168. Works for both from-tap and from-API installations.

To test from tap:

cd `brew --repo homebrew/cask`
git checkout 6b1f70c
brew install --cask ares
git checkout master
brew migrate --cask ares-emulator
brew uninstall --cask ares-emulator

Both ares-emulator folder and ares symlink will now be removed.

To test from API:

HOMEBREW_NO_INSTALL_FROM_API= brew install --cask angband-app
# in Caskroom, change to old name: folder, JSON filename in `.metadata`, JSON contents in 2 places
brew migrate --cask angband-app
brew uninstall --cask angband-app

[MikeMcQuaid]

Looks good, thanks @EricFromCanada! Test failure seems legit but good to merge once resolved.

[EricFromCanada]

Anyone have hints on how to fix the test?

[MikeMcQuaid]

@EricFromCanada Maybe needs a file written to disk? Might be worth seeing how old the test is.