Add a check for missing binaries to gdk doctor

Overview

We ran into an issue where gitlab-workhorse wasn't running in a fresh GDK because the installation process was likely aborted, and the binaries for gitlab-workhorse weren't downloaded. Now that GDK downloads pre-compiled binaries, we should add a check to gdk doctor to detect which binaries are present and flag any that are missing.

Impacted categories

The following categories relate to this issue:

• 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.

Steps to replicate (optional)

Proposal (optional)

Environment (optional)

• Operating system name: <!-- output of `uname -a` command -->
• Architecture: <!-- output of `arch` command -->
• The contents of your gdk.yml (if any)
• Ruby version: <!-- output of `ruby --version` command -->
• GDK version: <!-- output of `git rev-parse --short HEAD` command -->