Make cache key configurable

Currently, as far as I understand, the action uses `conan-${{ steps.install-conan.outputs.conan-version }}-${{ runner.os }}` (see https://github.com/conan-io/setup-conan/blob/main/action.yml#L112) as cache key. It would be even better to make it configurable by the users in order to support uses cases such as multiple configurations for the OS (e.g. different compilers, debug vs release etc).