Update cpu_inference checkout action

[loadams]

For some reason this was the only action that wasn't updated to use actions/checkout@v3. I see no reason why it couldn't, so this updates it to be in line.

Warning:

[unit-tests](https://github.com/microsoft/DeepSpeed/actions/runs/6356521056/job/17266306307)
The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/