Split compile jobs for macOS and Linux binaries

What does this merge request do and why?

Currently, we use a single job to compile binaries for both macOS and Linux. We recently found an issue where compiling Gitaly on macOS with Linux-specific flags caused some binaries to be built for macOS. To fix this, we'll separate the compile jobs for macOS and Linux to make sure the binaries are correctly built for each platform.

Related to #2228 (comment 2160776709)

How to set up and validate locally

For regular packages

1. Run bundle exec rake update:graphql
2. Make sure the GraphQL schema files are downloaded successfully

For platform-specfic packages

1. Run gdk config set gitaly.skip_compile true && support/package-helper gitaly download
2. Make sure the Gitaly binaries are downloaded correctly and match your platform. You can check by running file gitaly/_build/bin/*

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.