[llvm] Point workflow yaml at new docker image (PR #133359)

[George Burgess IV via llvm-commits]

gburgessiv wrote:

> I want to see what wins can be verified/bugs can be found outside-of-production.

- Initial pull/unpack of a docker image is regressed by about 60s on our builders. This is done once per builder at startup, so is likely neither here nor there.
- With a new LLVM checkout baked into the image, the 'sync' step sits around 15s.
- With a 5wk old LLVM checkout baked into the image, the 'sync' step sits around 32s.
- Without any LLVM checkout baked into the image, the 'sync' step sits around 75s.

So this saves somewhere from 40-60 seconds per run on Linux.

I'll test Win next, but shaving the better part of a minute of startup off in exchange for 60s of startup time sounds like a win to me. Sounds like we might be able to make this docker image part of our cloud image at some point, which should take the startup cost to near-0.

https://github.com/llvm/llvm-project/pull/133359