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

[George Burgess IV via llvm-commits]

gburgessiv wrote:

Hey, apologies for the latency here. It was unintentional, but I got signal on the implicit "what happens if this gets to be a month old" question - posted stats from experimenting here: https://github.com/llvm/llvm-project/pull/133359#issuecomment-2847434034. tl;dr from quick experimentation is that syncing goes from ~75s to ~15s when the container has a fresh checkout. When the checkout is 5wks old, syncing took closer to 30-35s.

> This definitely needs to be split

Done; if we're cool with this PR, I'll send https://github.com/llvm/llvm-project/pull/133359 for review once CI checks on it pass.

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