charmcraft: update to use pygit2 1.15.1

[chenrui333]

• 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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followup #166767

[carlocab]

We should just use the pygit2 formula here. We have that so that formulae that use pygit2 don't need to be recompiled whenever libgit2 is updated. CC @branchvincent

[chenrui333]

I am okay with either way

[branchvincent]

yup agreed these make sense

[github-actions]

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