message: fix outdated sysroot help message
diff: @@ -58,7 +58,7 @@ pub fn report_error<'tcx, 'mir>(
  };
  let help = match e.kind {
  Unsupported(UnsupportedOpInfo::NoMirFor(..)) =>
  - Some("set `MIRI_SYSROOT` to a Miri sysroot, which you can prepare with `cargo miri setup`"),
  + Some("make sure to use a Miri sysroot, which you can prepare with `cargo miri setup`"),
  Unsupported(_) =>
  Some("this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support"),
  UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_)) =>
  