spec: Clean up pre-existing dist/ in %prep

By convention.

See https://bugzilla.redhat.com/show_bug.cgi?id=1969450

packaging/cockpit-starter-kit.spec.in

@@ -27,10 +27,10 @@ Cockpit Starter Kit Example Module
 %prep
 %setup -q -n %{name}
 %setup -q -a 1 -n %{name}
-
-%build
 # ignore pre-built webpack in release tarball and rebuild it
 rm -rf dist
+
+%build
 ESLINT=0 NODE_ENV=production make
 
 %install