Rollup merge of #131442 - jieyouxu:mir-opt-rebuild, r=onur-ozkan

Match std `RUSTFLAGS` for host and target for `mir-opt` test suite to fix double std build/rebuilds

Previously the bootstrap compiletest `Step::run` flow had:

```rs
// ensure that `libproc_macro` is available on the host.
builder.ensure(compile::Std::new(compiler, compiler.host));

// ...

if suite == "mir-opt" {
    builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, target));
} else {
    builder.ensure(compile::Std::new(compiler, target));
}
```

This can cause unnecessary std rebuilds (even on the same invocation) because if host == target then `builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, target))` will have different `RUSTFLAGS` than `builder.ensure(compile::Std::new(compiler, compiler.host))`.

This PR fixes that by matching up std `RUSTFLAGS` if the test suite is `mir-opt`:

```rs
if suite == "mir-opt" {
    builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, compiler.host));
} else {
    builder.ensure(compile::Std::new(compiler, compiler.host));
}
```

This is a short-term fix, the better fix is to enforce how `RUSTFLAGS` are handled as described in #131437 (comment).

Fixes #131437.

src/bootstrap/src/core/build_steps/test.rs

@@ -1697,7 +1697,11 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         builder.ensure(TestHelpers { target: compiler.host });
 
         // ensure that `libproc_macro` is available on the host.
-        builder.ensure(compile::Std::new(compiler, compiler.host));
+        if suite == "mir-opt" {
+            builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, compiler.host));
+        } else {
+            builder.ensure(compile::Std::new(compiler, compiler.host));
+        }
 
         // As well as the target
         if suite != "mir-opt" {