Rollup merge of rust-lang#145291 - Zalathar:no-warning, r=Kobzol

bootstrap: Only warn about `rust.debug-assertions` if downloading rustc

The changes in rust-lang#145149 had the unwanted side-effect of causing bootstrap to *always* warn about `rust.debug-assertions = true`, even if rustc isn't going to be downloaded anyway.

cc ``@Shourya742`` ``@Kobzol``

Author: Zalathar

src/bootstrap/src/core/config/config.rs

@@ -955,7 +955,7 @@ impl Config {
         config.download_rustc_commit =
             download_ci_rustc_commit(dwn_ctx, rust_download_rustc, config.llvm_assertions);
 
-        if debug_assertions_requested {
+        if debug_assertions_requested && config.download_rustc_commit.is_some() {
             eprintln!(
                 "WARN: `rust.debug-assertions = true` will prevent downloading CI rustc as alt CI \
                 rustc is not currently built with debug assertions."