Update GitHub actions instructions

The instructions at https://intuit.github.io/auto/docs/build-platforms/github-actions still mention `actions/setup-node@v1` even though version 3 is out already. The new version also supports caching so we probably don't need a separate step. The same goes for `actions/checkout@v2` which has a version 3 already. It also has `fetch-depth: 0`, which may be able to replace the extra git step in the instructions.