Fix Miri sysroot for x run

Miri no longer respects `MIRI_SYSROOT` and wants to be treated like a
REAL rustc, with `--sysroot`. *pats Miri* sure Miri, just for you :3.

src/bootstrap/src/core/build_steps/run.rs

@@ -154,7 +154,7 @@ impl Step for Miri {
         miri.args(builder.config.args());
 
         // miri tests need to know about the stage sysroot
-        miri.env("MIRI_SYSROOT", &miri_sysroot);
+        miri.arg("--sysroot").arg(miri_sysroot);
 
         let mut miri = Command::from(miri);
         builder.run(&mut miri);