resolve stage0 sysroot from rustc

Instead of manually navigating directories based on stage0 rustc, use `--print sysroot`
to get the sysroot directly. This also works when using the bootstrap `rustc` shim.

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

Author: onur-ozkan

src/bootstrap/src/core/config/config.rs

@@ -1832,7 +1832,9 @@ impl Config {
                 .join(exe("rustc", config.build))
         };
 
-        config.initial_sysroot = config.initial_rustc.ancestors().nth(2).unwrap().into();
+        config.initial_sysroot = t!(PathBuf::from_str(
+            output(Command::new(&config.initial_rustc).args(["--print", "sysroot"])).trim()
+        ));
 
         config.initial_cargo_clippy = cargo_clippy;