Previously, the script was caching any file already present in the
destination directory, regardless of its origin. This caused issues
when the directory contained files copied from external sources like
`autoreconf --install`.

For example:
1. `./autogen.sh --install` copies `config.guess` and `config.sub`
   from the system to `./tool`.
2. `ruby tool/downloader.rb -d tool -e gnu config.guess config.sub`
   treats those files as if they were downloaded and caches them.
3. Removing the files: `rm tool/config.guess tool/config.sub`.
4. Running the downloader again, it mistakenly restores the cached
   files instead of downloading fresh versions.

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

Move `Downloader::Unicode` specific options, and parse options after
the downloader specificier.

zlib.net rarely returns the current time in RFC 2616 noncompliant
format in the response header, and the checksum does not match in
that case (maybe creating the tarball on the fly?).