explain why we use in-tree std for compiletest

Signed-off-by: onur-ozkan <work@onurozkan.dev>

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

@@ -686,6 +686,8 @@ fn run(self, builder: &Builder<'_>) {
         let mut cargo = tool::prepare_tool_cargo(
             builder,
             compiler,
+            // compiletest uses libtest internals; make it use the in-tree std to make sure it never breaks
+            // when std sources change.
             Mode::ToolStd,
             host,
             "test",