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). [jc: cherry-picked from v2.45.0-2-g11c7001e3d] Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>