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
```
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