spec: Set ExclusiveArch:

This forces the package build to happen on a koji builder which actually
supports node. It's pointless at runtime (as the code runs in a browser,
not through node), but in practice, `nodejs_arches` encompasses all
supported Fedora/RHEL architectures anyway.

https://docs.fedoraproject.org/en-US/packaging-guidelines/Node.js/#_exclusivearch

packaging/cockpit-starter-kit.spec.in

@@ -7,6 +7,7 @@ License: LGPLv2+
 Source0: https://github.com/cockpit-project/starter-kit/releases/download/%{version}/%{name}-%{version}.tar.xz
 Source1: https://github.com/cockpit-project/starter-kit/releases/download/%{version}/%{name}-node-%{version}.tar.xz
 BuildArch: noarch
+ExclusiveArch: %{nodejs_arches} noarch
 %if 0%{?fedora}
 BuildRequires: nodejs-devel
 %endif