Ability to use different compressor

[Fuuzetsu]

Currently we are forced to use gunzip to make tar.gz archives.

I'd like to use zstd because it's just better: notably, it's much faster. This can make a big difference in CI when syncing down packages.

I think if we are able to give flags like

• --compressor=zstd
• --compressor-extension=zst

The changes to code ought to be fairly minimal:

• When compressor is set, pass -I <compressor> to tar.
• Instead of tar.gz, use tar.<compressor-extension>. gz if not set. Don't pass -z to tar unless we're using gunzip.

Maybe instead of --compressor, multiple --tar-flag are much more flexible.

This approach relies on the compressor binary being installed and doesn't add any new dependency to cabal-cache itself.

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Apparently tar itself has some zstd support via the --zstd flag but at least on my system, but it's probably a much newer flag than just using -I.