[PATCH 3/3] ci: stop installing "gcc-13" for osx-gcc

Our osx-gcc job explicitly asks to install gcc-13. But since the GitHub runner image already comes with gcc-13 installed, this is mostly doing nothing (or in some cases it may install an incremental update over the runner image). But worse, it recently started causing errors like:

    ==> Fetching gcc@13
    ==> Downloading https://ghcr.io/v2/homebrew/core/gcc/13/blobs/sha256:fb2403d97e2ce67eb441b54557cfb61980830f3ba26d4c5a1fe5ecd0c9730d1a
    ==> Pouring gcc@13--13.2.0.ventura.bottle.tar.gz
    Error: The `brew link` step did not complete successfully
    The formula built, but is not symlinked into /usr/local
    Could not symlink bin/c++-13
    Target /usr/local/bin/c++-13
    is a symlink belonging to gcc. You can unlink it:
      brew unlink gcc

which cause the whole CI job to bail.

I didn't track down the root cause, but I suspect it may be related to homebrew recently switching the "gcc" default to gcc-14. And it may even be fixed when a new runner image is released. But if we don't need to run brew at all, it's one less thing for us to worry about.

Signed-off-by: Jeff King <peff@peff.net>
---
I don't think this was due to anything on our side. I tried to re-run
versions of git.git which had previously passed and ran into the same
issue.

I'd like to report that this let me get a successful CI run, but I'm running into the thing where osx jobs seem to randomly hang sometimes and hit the 6-hour timeout. But I did confirm that this lets us get to the actual build/test, and not barf while installing dependencies.

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