[Skiboot] [PATCH] ci: Update github actions to run on Node 16

[Reza Arbab]

On Wed, Jan 04, 2023 at 01:35:58PM -0600, Reza Arbab wrote:
>Github is deprecating Node 12 for actions[1]. Update our workflow from
>using actions/checkout at v2 to v3, which runs on Node 16.

Applied to master.

-- 
Reza Arbab