Prefer `process.env.XDG_CACHE_HOME` on macOS

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If a user explicitly defines an XDG_CACHE_HOME, it should be used. This is already the case on posix systems:

function posix (id) {
  const cacheHome = process.env.XDG_CACHE_HOME || path.join(os.homedir(), '.cache')
  return path.join(cacheHome, id)
}

It would make sense to do the same on darwin/macOS:

function darwin (id) {
  const cacheHome = process.env.XDG_CACHE_HOME || path.join(os.homedir(), 'Library', 'Caches')
  return path.join(cacheHome, id)
}