LLVM update detection only works with bors merge commit

[nikic]

LLVM update detection in bootstrap currently only works if there is a bors merge commit:

rust/src/bootstrap/native.rs

Line 134 in d610b0c

format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),

This means that it's not possible to run docker images with updated LLVM locally. For dist images you can use DEPLOY=1, but for non-dist images we always download CI LLVM. You can't even specify NO_DOWNLOAD_CI_LLVM=1 without patching the Dockerfile or run.sh.

LLVM updates should be detected locally, without a bors merge commit as well.