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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

> Meta: I'm not familiar with the actual deployment process here, but I'd ideally like to be able to test this PR prior to submitting it. Not sure whether that'll be worth the effort compared to landing it (or splitting it, waiting for the new Docker image to appear, then messing with the premerge.yaml part of this) though?

This definitely needs to be split. The deployment isn't really atomic with the new container needing a little bit to build and then roll out. So if you land the change in `premerge.yaml` at the same time, you're likely to end up with a bunch of build failures assuming the patch has a hard dependency. In this case it looks like it might be fine given it should just clone to the newly specified directory, but keeping them separate is probably best practice.

In order to test it, you need to build the image locally, push it to a container registry somewhere (GHCR on your personal github account I think should work), and then point `premerge.yaml` at that container, which should let you test it.

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