[sbt2] Consider automatically caching the machine-wide cache

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