Prefer `process.env.XDG_CACHE_HOME` on macOS

Nobody has claimed this yet.

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

```js
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:

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

Start at the darwin(id) cache-directory implementation described in the issue and compare it with the posix(id) behavior. Verify that an explicitly set XDG_CACHE_HOME is preferred on macOS while the existing Library/Caches fallback remains unchanged.

Written by the indexing model from the issue text.