zstd usually offers better compression ratios, but also offers much
higher decompression speeds, which can tangibly impact image unpack
times. Enable this by default given it is a two line change that is
transparent at image pull time given most container runtimes these days
supported zstd compressed images.

This allows for their reuse inside llvm-zorg. Otherwise we get an error
that we cannot use reusable workflows inside job steps because Github
thinks that they are reuseable workflows rather than composite actions.
This should be NFC inside the monorepo.