CI: Improve caching

[okafke]

- name: Cache build
  id: cache
  uses: actions/cache@v5
  with:
    path: ${{ inputs.dir }}/build
    key: build-${{ inputs.mc }}-${{ hashFiles(format('{0}/[a-z]**', inputs.dir), 'api/**') }}

Whenever release-please updates the gradle.properties within the api folder this will cause every single version to be rebuilt, just for bumping one version string. However, if the code has not changed we would only have to take the cached jars from the previous release, rename their version to the next version and maybe fix the version string in files inside the jar (fabric.mod.json etc.).