LinuxContainer: finish the copyOut metadata stream on every path (#837)

metadataCont.finish() only runs inside onMetadata, which vminitd.copy
never invokes when the guest path is missing.

Sources/Containerization/LinuxContainer.swift

@@ -1356,6 +1356,7 @@ extension LinuxContainer {
 
             try await withThrowingTaskGroup(of: Void.self) { group in
                 group.addTask {
+                    defer { metadataCont.finish() }
                     try await state.vm.withAgent { agent in
                         guard let vminitd = agent as? Vminitd else {
                             throw ContainerizationError(.unsupported, message: "copyOut requires Vminitd agent")