Add `gh` CLI to runner container images

[wyardley]

What would you like added?

While I understand some of the reasons in the previous issue about this, I'm trying to open a new issue to help make sure this doesn't get lost track of.

The regular GH runners have gh pre-installed, and not having it on self-hosted runners by default is inconvenient at best.

While it's not a huge number of lines to install it, it is somewhat non-trivial, and there are a few (poorly maintained) third party actions to install it

Why is this needed?

This would provide better parity with GitHub's own hosted runners

Additional context

See #1366

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