Rollup merge of #110113 - jyn514:download-rustc-cross, r=albertlarsan68

Fix `x test ui --target foo` when download-rustc is enabled

Previously, we would never build the target std, only the host std:

```
; x t tests/ui/attributes --target wasm32-unknown-unknown
Building bootstrap
    Finished dev [unoptimized] target(s) in 0.02s
Building stage0 library artifacts (x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 0.08s
Building tool compiletest (stage0)
    Finished release [optimized] target(s) in 0.09s
Check compiletest suite=ui mode=ui (x86_64-unknown-linux-gnu -> wasm32-unknown-unknown)
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { depth: 0, inner: Io { path: Some("/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/wasm32-unknown-unknown/lib"), err: Os { code: 2, kind: NotFound, message: "No such file or directory" } } }', src/tools/compiletest/src/main.rs:842:31
```

Helps with #81930.

Author: compiler-errors

src/bootstrap/compile.rs

@@ -83,11 +83,11 @@ impl Step for Std {
         let target = self.target;
         let compiler = self.compiler;
 
-        // These artifacts were already copied (in `impl Step for Sysroot`).
-        // Don't recompile them.
+        // When using `download-rustc`, we already have artifacts for the host available
+        // (they were copied in `impl Step for Sysroot`). Don't recompile them.
         // NOTE: the ABI of the beta compiler is different from the ABI of the downloaded compiler,
         // so its artifacts can't be reused.
-        if builder.download_rustc() && compiler.stage != 0 {
+        if builder.download_rustc() && compiler.stage != 0 && target == builder.build.build {
             return;
         }