feat(setup-r-dependencies): support dependencies: 'FALSE'

[eitsupi]

This change will allow the following, which do not install dependencies and only want to install extra-packages.
Something like:

      - uses: r-lib/actions/setup-r-dependencies@v2
        with:
          dependencies: 'FALSE'
          extra-packages: any::pkgload

[gaborcsardi]

Thanks!

[github-actions]

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