Auto merge of rust-lang#96000 - jyn514:faster-doctests, r=Mark-Simulacrum

Make `x test --stage 2 compiler/rustc_XXX` faster to run

Previously, bootstrap unconditionally rebuilt the stage 2 compiler,
even if it had previously built stage 1. This changes it to reuse stage 1 if possible.
In particular, it no longer runs the following step:
```
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu(x86_64-unknown-linux-gnu) -> x86_64-unknown-linux-gnu(x86_64-unknown-linux-gnu))
```

Author: bors

src/bootstrap/test.rs

@@ -1892,7 +1892,8 @@ impl Step for CrateLibrustc {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.build_triple());
+        let host = run.build_triple();
+        let compiler = builder.compiler_for(builder.top_stage, host, host);
         let krate = builder.crate_paths[&run.path];
         let test_kind = builder.kind.into();
 
@@ -1929,7 +1930,8 @@ impl Step for Crate {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.build_triple());
+        let host = run.build_triple();
+        let compiler = builder.compiler_for(builder.top_stage, host, host);
         let test_kind = builder.kind.into();
         let krate = builder.crate_paths[&run.path];