[CI] Bump actions/checkout to v3

Summary: > Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout, actions/checkout

Test Plan: This is a first party GitHub dependency so it should be fine.. but we'll see after landing.

Reviewers: tomek, varun, marcin, ashoat, ginsu, rohan

Reviewed By: ashoat

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5550