Cache SwiftDoc binary on Cirrus CI

.cirrus.yml

task:
  name: Test Swift-Doc 🎉
  osx_instance:
    image: catalina-xcode
  env:
    SWIFT_DOC_VERSION: 1.0.0-beta.3
    PATH: $PATH:$CIRRUS_WORKING_DIR/.cache/swift-doc-bin/
    
  swiftdoc_install_cache:
    folder: .cache/swift-doc-bin
    fingerprint_script: echo $SWIFT_DOC_VERSION
    populate_script:
      - git clone --branch $SWIFT_DOC_VERSION https://github.com/SwiftDocOrg/swift-doc.git
      - pushd swift-doc
      - make install prefix=build
      - popd
      - mkdir -p .cache/swift-doc-bin
      - mv swift-doc/build/bin/swift-doc .cache/swift-doc-bin/swift-doc
      - rm -rf swift-doc    
  swiftdoc_script: swift-doc --version