lean 3.18.4

[pthariensflame]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Updates URLs to the new home of Lean 3 , and adds lean@3 alias in preparation for Lean 4.

[SMillerDev]

Unless there proves to be sufficient demand and support we won't be making a versioned formula so please remove the alias.

[pthariensflame]

@SMillerDev Done!

[BrewTestBot]

🤖 A scheduled task has triggered a merge.

[semorrison]

@pthariensflame, I'm not sure it makes sense have formulas for Lean itself. Versions come out pretty rapidly, and we have a great tool elan for automatically providing the best version. Providing people with the capability to get Lean via brew install lean seems almost a disservice -- a week later they're stuck!

[pthariensflame]

@semorrison Do you think it would make sense to at least provide elan as a Homebrew formula? For comparison, both rust and rustup are.