Upgrade CI

[gnclmorais]

I wrote Upgrade instead of fix ‘cause I still have a couple of issues to look into, but for now this will help follow-up work:

• Add workflow_dispatch to manually trigger builds (I’ve found that useful for certain debugging scenarios)
• Change Ubuntu version to latest (there is nothing particular we gain from pinning down out distro version)
• Bump Node actions to v2 and cache Yarn through them
• Set fail-fast: false to make sure all of our parallel builds run (instead of having them being cancelled because one failed)

After having these improvements merged in, we can look into how to fix the following (modern Ember versions don’t seem to play well with this addon):