chromiumBeta: Fix the build

We do already set `system_wayland_scanner_path` to
`"${wayland}/bin/wayland-scanner"` but apparently wayland-scanner wasn't
required (anymore?) as wayland-scanner is only in the `bin` output (I have a
few ideas what could've changed but didn't bother to check as it isn't worth
the time as long as it works now).

This fixes the following build error:
```
ninja: error: '../../../../../../../../nix/store/l3y9k2x7cqzcjj9s18z7la9xqsjq6r52-wayland-1.21.0/bin/wayland-scanner', needed by 'gen/components/exo/wayland/protocol/aura-shell-protocol.c', missing and no known rule to make it
```

pkgs/applications/networking/browsers/chromium/common.nix

@@ -303,6 +303,7 @@ let
       # as well to avoid incompatibilities (if this continues to be a problem
       # from time to time):
       use_system_libwayland = true;
+      system_wayland_scanner_path = "${wayland.bin}/bin/wayland-scanner";
     } // optionalAttrs proprietaryCodecs {
       # enable support for the H.264 codec
       proprietary_codecs = true;