Resolve "Add basic timing to gdk install/update"

What does this Merge Request do and why?

This MR adds a summary at the end of a gdk update to show how long each target is taking to execute in seconds, e.g.

$ gdk update
--snip--

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Timings
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preflight-checks: 5 sec(s)
preflight-update-checks: 2 sec(s)
asdf-update: 57 sec(s)
gitlab-git-pull: 18 sec(s)
gitlab-shell-update: 10 sec(s)
gitlab-shell-git-pull: 5 sec(s)
gitlab-workhorse-update: 9 sec(s)
gitlab-pages-update: 5 sec(s)
gitaly-update: 22 sec(s)
gitaly-git-pull: 3 sec(s)
gitlab-update: 145 sec(s)
object-storage-update: 1 sec(s)
jaeger-update: 1 sec(s)
gitlab-docs-update: 39 sec(s)

Took 368 sec(s) total.

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Updated successfully as of 2021-06-17 12:39:51
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There's some overhead and other targets that aren't captured which is why the individual timings added up don't equal the total at the end 😉

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 Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1201 (closed)