Enable std/panic-unwind when checking std

`std/panic-unwind` gets enabled when building the sysroot, but not when
checking `std`. This was not a problem with the v1 resolver because it
would unify features and just always enable `panic-unwind`.

With the v2 resolver, however, this causes `std` to get built twice with
different sets of features. This then causes an "multiple candidates for
`rmeta` dependency `std` found" error.

Always enable the `panic-unwind` feature to avoid this conflict.

src/bootstrap/src/core/build_steps/check.rs

@@ -67,6 +67,7 @@ impl Step for Std {
             target,
             self.override_build_kind.unwrap_or(builder.kind),
         );
+        cargo.arg("--features=std/panic-unwind");
 
         std_cargo(builder, target, compiler.stage, &mut cargo);
         if matches!(builder.config.cmd, Subcommand::Fix { .. }) {