Auto merge of #146771 - Kobzol:download-ci-llvm-default-value, r=jieyouxu

Simplify default value of `download-ci-llvm`

Just set it to true, rather than having different default values on CI and locally, and then only deny `true` on our own CI, not elsewhere.

Closes: rust-lang/rust#146768

r? `@jieyouxu`

Author: bors