Auto merge of #126153 - onur-ozkan:fix-rustdoc-issue-with-ci-rustc, r=Mark-Simulacrum

resolve rustdoc incompatibility with `rust.download-rustc=true` + `rust.channel= beta/stable`

Previously, we were unable to use `rust.download-rustc` with the beta or stable
channel settings through `rust.channel` due to breaking rustdoc UI tests.
This was because when using a precompiled nightly compiler from CI, we must use the
channel of precompiled compiler and ignore `rust.channel` from the configuration.

This change addresses that issue in `Builder::doc_rust_lang_org_channel` and allows rustdoc
UI tests to work with the precompiled compiler even if the channel specified in config.toml is
"beta" or "stable".

Blocker for rust-lang/rust#122709

Author: bors