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.

[jc: cherry-picked from v2.45.0-3-g7df2405b38]

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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