Improve CI build times by fixing caching

[DamianReeves]

Now that the CI has switched over to using mill we need to properly cache the things mill cares about.

Change the caching config in ci.yaml to cache mill, coursier, and ivy resources:

Things to cache:

• ~/.ivy/cache
• ~/.m2
• ~/.cache/coursier
• ~/.cache/mill
• out

These paths probably need to be relative to the runner.

Info on cache setup can be found here:
https://github.com/actions/cache/blob/master/examples.md#scala---sbt