Auto merge of #131453 - onur-ozkan:llvm-ci-check-for-rustc-ci, r=Kobzol

disable `download-rustc` if LLVM submodule has changes in CI

We can't use CI rustc while using in-tree LLVM (which happens in LLVM submodule update PRs) and this PR handles that by ignoring CI-rustc in CI and failing in non-CI environments.

Author: bors