The existing actions/checkout@v2 is causing Node v12 deprecation warnings
to be shown in GitHub Actions. v3 uses Node v16, which will stop the
warnings.
Changes are:
- Use ubuntu-20.04 so that gcc-multilib installs without error.
- Use "fetch-depth: 100" to get history prior to pull request.
Signed-off-by: Damien George <damien@micropython.org>
