generate-cask-ci-matrix: fix incorrect error being raised

[bevanjkay]

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The comparison between the working directory and tap path did not function correctly on GitHub actions, because we don't checkout the tap to a standard location when using Setup Homebrew.
The command must be run from within the tap directory, which is the default path in GitHub actions, so this should not be an issue.

[MikeMcQuaid]

Thanks @bevanjkay!