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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> How often would we update the git cache?

I was thinking about as often as we currently build the container (whenever patches land). Running a git fetch even on a checkout that is a month or two old only takes something like ten seconds from my experience, and should always be much quicker than doing the full (shallow) checkout that we do currently.

If it becomes an issue and we want to update it more frequently, it should be trivial to split building the main container from building the agent container. We could then build the agent container on the free Github hosted runners weekly or something depending upon what ends up being best.

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