Update actions/checkout to v3 in GitHub Actions CI workflow

[esabol]

This trivial pull request partially addresses issue #354. It eliminates the following warnings currently emitted by the GitHub Actions CI workflow:

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@v2

All it does is change uses: actions/checkout@v2 to uses: actions/checkout@v3.

[SpamapS]

Thanks!