Add ability to skip gitaly, workhorse and shell setup via env variables

What does this merge request do and why?

Allows to skip several component builds (gitaly, workhorse and gitlab-shell) based on environment variable value.

This is needed in gitlab project because we build these components separately during docker build and rely on docker caching to not rebuild them if relevant files have not changed but we still run make all task as final instruction to set up gdk which otherwise triggers these builds again even if the compiled files are present.

Related to: gitlab#410241 (closed)

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.