fix Builder::doc_rust_lang_org_channel

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 (which is nightly) 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".

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

src/bootstrap/src/core/builder.rs

@@ -1039,6 +1039,10 @@ impl<'a> Builder<'a> {
 
     pub fn doc_rust_lang_org_channel(&self) -> String {
         let channel = match &*self.config.channel {
+            // When using precompiled nightly compiler from CI, we need to use CI rustc's channel and
+            // ignore `rust.channel` from the configuration. Otherwise most of the rustdoc tests will fail
+            // due to incompatible `DOC_RUST_LANG_ORG_CHANNEL`.
+            _ if self.download_rustc() => "nightly",
             "stable" => &self.version,
             "beta" => "beta",
             "nightly" | "dev" => "nightly",