[sbt2] Consider automatically caching the machine-wide cache

[keynmol]

Now that the sbt2 cache is machine wide, we can pre-configure Github Actions cache, e.g for macos.

      - name: Cache SBT
        uses: actions/cache@v5
        with:
          path: |
            ~/Library/Caches/sbt/v2
          key: ${{ matrix.OS }}-sbt2

I have confirmed that this works and subsequent builds skip compilation

[eed3si9n]

I think this is a good idea. Given that setup-java already has cache: sbt, I wonder if we should first attempt to add the directory on setup-java side.

- name: Setup JDK
  uses: actions/setup-java@v5
  with:
    distribution: temurin
    java-version: 17
    cache: sbt

[keynmol]

That's a good idea, will be more explicit opt in and less controversial than implementing caching in setup-sbt.

I'll have a look at setup-java and see what needs to be changed

[eed3si9n]

I should've read setup-java's doc carefully https://github.com/actions/setup-java#caching-packages-dependencies says

Caching packages dependencies

The action has a built-in functionality for caching and restoring dependencies. .... where the hash is based on the following files: ... **/*.sbt

So while setup-java does offer caching, it's primarily for caching of the built tool and the 3rdparty librararies, i.e. Coursier cache, I think.

So I guess sbt 2.x disk-cache can be in setup-sbt. Speaking of explicit opting in or out, we can provide something like:

- uses: sbt/setup-sbt@v1
  with:
    disk-cache: false