[llvm] [Github][CI] Add separate container for code-format premerge job (PR #161083)

[Aiden Grossman via llvm-commits]

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+FROM docker.io/library/ubuntu:24.04 AS llvm-downloader
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boomanaiden154 wrote:

This can just be `ubuntu:24.04`. Most container runtimes will by default use `docker.io` as the default registry, and that's the convention we use everywhere else.

https://github.com/llvm/llvm-project/pull/161083