cargo-llvm-cov: use rustup

[branchvincent]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 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 HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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Split from #177840

[github-actions]

Hi, thanks for your contribution to Homebrew! You already have >=15 open pull requests, so please get them ready to be merged or close them before you open more. If CI fails on any of them, please fix it or ask for help doing so.
If you are performing simple version bumps, @BrewTestBot automatically bumps a list of formulae so you don't need to. Please take a look at issues and pull requests labelled help wanted Task(s) needing PRs from the community or maintainers and see if you can help to fix any of them. Thanks!