Fix CGO_CFLAGS to use output from `brew --prefix`
When testing GDK on a MacOS running on Intel processor, we can't compile gitlab-shell because it has hardcoded a location for Homebrew that is not the default one on Intel processor.
Instead we should rely on homebrew itself to tell its installation location via: brew --prefix.
You can see the failing build here: https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/4075185632
Edited by Gabriel Mazetto