Improvements to GitHub Actions CI setup

**Type of issue**: feature request

**Impact**: no functional change

**Development Phase**: request

**Other information**

Now that https://github.com/chipsalliance/rocket-chip/pull/2465 has merged in, we have GitHub Actions CI enabled, but there are a few things that could be improved about the experience:

- In our README_GITHUB_ACTIONS, we could document how to rerun a failed job, and explicitly mention that it is not possible to rerun just a single job and that you must rerun the whole workflow.
- Restoring from the cache seems to be somewhat flaky, and individual jobs may fail to do that. If we install the full set of dependencies for compiling riscv-tools and Verilator, we can at least let the jobs succeed even if the cache restoration fails, albeit more slowly. GitHub Actions has a 5 hour timeout compared to Travis's default of 2 hours, so even if we have to rebuild riscv-tools in an individual job, it'll still have enough time to complete
- Let's add the CI status badge to the README!

Others who are experiencing issues are free to add more requests to this ticket.