Add basic timing to gdk install/update

Overview

Currently it's not clear how long each section of a gdk install/update takes, making it hard to know if the time required to run is taking longer (I suspect it is).

Proposal

In order to ensure we're making progress on making the GDK easier and faster to install/update, we should add basic timing to:

• A total elapsed time at the end
• Break down elapsed time at the end for each section
• Each section, e.g. Building git for gitlab-org/gitaly
• Potentially a summary at the end also, showing each section and it's total time taken