[llvm] [github] pre-cache llvm-project in Linux CI Docker images (PR #133137)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 requested changes to this pull request.

This needs to at least be split into a separate build stage in the container so that it can be done in a separate job.

It's probably easiest to create a new Dockerfile that pulls the agent image (might be a good idea to pull the agent bits out too), adds the cloned repo, and then uploads again. We don't want to have to rebuild the whole toolchain on multiple arches everyimt we want to refresh the repo, which we probably want to do with a cron job.

https://github.com/llvm/llvm-project/pull/133137