[llvm] [Github] Make Windows container use zstd (PR #167022)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> One drawback I see is the increasing pool of tools we use. This can bring us more potential bugs that could appear during version bumps. Also, entry barrier is slightly increased with introduction of scopeo.

The versions here are managed by the stable release distros since we just install from the package manager. `skopeo` is also pretty simple. If we couldn't use `skopeo`, I would probably end up writing my own python script to manually repack the images with zstd, but this makes it a lot simpler. The full implementation of that I think would be a couple hundred lines of Python. Instead we get to use a well supported tool that is part of the standard container ecosystem. It would be pretty pretty weird if a version bump of skopeo broke our workflows given what it does is not super complicated.

It's also a much simpler tool than anything else that we're using, especially for this use case. It reads a JSON blob, modifies some objects in a CAS, and then pushes an archive to a server. I don't think the entry barrier tangibly changes.

https://github.com/llvm/llvm-project/pull/167022