[Skiboot] [PATCH] ci: Update github actions to run on Node 20

[Reza Arbab]

On Fri, Aug 02, 2024 at 01:29:41PM -0500, Reza Arbab wrote:
>Github is deprecating Node 16 for actions[1]. Update our workflow from
>using actions/checkout at v3 to v4, which runs on Node 20.

Applied to master.

-- 
Reza Arbab