mullvadvpn: fetch from github

[yasinuslu]

Notes

I've noticed that the mullvadvpn's cdn is really slow (maybe it is slow in my region, not sure). Thought it would be better to use their artifact in github instead.

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Checkboxes

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
  • I have trouble running this because I use nix-darwin
  • I am checking the box because CI seemed to have ran that with no issues on both intel and arm
• brew style --fix <cask> reports no offenses.

[samford]

I'll leave the decision of whether to switch the cask url source up to the cask maintainers but I've added a livecheck block suggestion in case we go forward with this.

For what it's worth, I can't remember if this is a consistent issue or just a short-term fluke but the most recent version was noticeably slow to download for me as well.

[khipp]

Thank you!