downloader.rb: unlink broken symlink before caching to fix ENOENT

On GitHub CI, the downloader's cache is shared across different jobs.
For some systems, the cached config.guess ends up being a symlink to
/usr/share/autoconf/build-aux/config.guess and containers that don't
have that file end up consuming the cache anyways, leading to ENOENT
when trying to cache the downloaded file.

This error happened on forks:
 - https://github.com/XrXr/ruby/actions/runs/5675262636/job/15380232344
 - https://github.com/peterzhu2118/ruby/actions/runs/5684765421/job/15408188728

Author: XrXr

tool/downloader.rb

@@ -270,6 +270,7 @@ def self.download(url, name, dir = nil, since = true, options = {})
     end
     dest = (cache_save && cache && !cache.exist? ? cache : file)
     dest.parent.mkpath
+    dest.unlink if dest.symlink? && !dest.exist?
     dest.open("wb", 0600) do |f|
       f.write(data)
       f.chmod(mode_for(data))