Fix tool cargo being off-by-one from rustc staging

Previously if you pass compiler stage 1 to `tool::Cargo`, it will build
stage2 rustc and give you back a cargo built with stage2 rustc, which is
not what we want.

This commit adds a hack that chops off a stage from the compiler passed
to `tool::Cargo`, meaning that we will get a cargo built with stage 1
compiler, avoiding unnecessary and incorrect build of stage2 rustc and
the cargo built by that.

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

@@ -1735,6 +1735,14 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 builder.initial_cargo.clone()
             } else {
                 // We need to properly build cargo using the suitable stage compiler.
+
+                // HACK: currently tool stages are off-by-one compared to compiler stages, i.e. if
+                // you give `tool::Cargo` a stage 1 rustc, it will cause stage 2 rustc to be built
+                // and produce a cargo built with stage 2 rustc. To fix this, we need to chop off
+                // the compiler stage by 1 to align with expected `./x test run-make --stage N`
+                // behavior, i.e. we need to pass `N - 1` compiler stage to cargo. See also Miri
+                // which does a similar hack.
+                let compiler = builder.compiler(builder.top_stage - 1, compiler.host);
                 builder.ensure(tool::Cargo { compiler, target: compiler.host })
             };