ci: avoid bare "gcc" for osx-gcc job

ci: avoid bare "gcc" for osx-gcc job

On macOS, a bare "gcc" (without a version) will invoke a wrapper for
clang, not actual gcc. Even when gcc is installed via homebrew, that
only provides version-specific links in /usr/local/bin (like "gcc-13"),
and never a version-agnostic "gcc" wrapper.

As far as I can tell, this has been the case for a long time, and this
osx-gcc job has largely been doing nothing. We can point it at "gcc-13",
which will pick up the homebrew-installed version.

The fix here is specific to the github workflow file, as the gitlab one
does not have a matching job.

It's a little unfortunate that we cannot just ask for the latest version
of gcc which homebrew provides, but as far as I can tell there is no
easy alias (you'd have to find the highest number gcc-* in
/usr/local/bin yourself).

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