Add caching to the setup-java GitHub Action for Mill, specifically

[clintval]

Hello! I use Mill a lot for Scala projects and hope to speedup GitHub Actions that use Mill by caching important files. I also use the setup-java GitHub Action to easily test a matrix of JDKs/JREs in projects. I noticed the GitHub Action was lacking built-in Mill cache support so I opened a draft PR here:

• Add caching for the Mill build tool actions/setup-java#819

I am wondering if others see value in this small feature enhancements and I'm also wondering from those more sophisticated with Mill if I am caching the most relevant files and am also refreshing the cache correctly when other relevant files change (such as build.sc).

Anyway, let me know what you think and if you see something I missed!