fix: do not inherit xen extension for specialisation

Makes it possible to have system specialisation without xen, but now,
only the toplevel system derivation can have xen installed, xen
extension is always ignored for specialisations.

Issue: DeterminateSystems/bootspec#147

rust/tool/shared/src/generation.rs

@@ -102,7 +102,10 @@ impl Generation {
             spec: ExtendedBootJson {
                 bootspec: bootspec.clone(),
                 lanzaboote_extension: self.spec.lanzaboote_extension.clone(),
-                xen_extension: self.spec.xen_extension.clone(),
+                // TODO: It is only possible to use xen with toplevel system derivation, as there is no
+                // extension fields for bootspec generation anymore:
+                // https://github.com/DeterminateSystems/bootspec/issues/147
+                xen_extension: None,
             },
             ..self.clone()
         }