Migrate uhk-agent to homebrew-cask

[daniels]

I use this. Helpful for my bootstrap scripts if it can remain.

(It was already deleted from homebred-cask-drivers in #3443).

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In the following questions <cask> is the token of the cask you're submitting.

After making any changes to a cask, existing or new, verify:

• [x] The submission is for a stable version or documented exception.
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