run_unix: don't return an error from getNetworkInterface

While networking is not supported on non-Linux systems, many
operations can still succeed if we allow buildah to proceed without a
NetworkInterface object.

To do so, don't return an error from getNetworkInterface.

Signed-off-by: Sergio Lopez <slp@redhat.com>

run_unix.go

@@ -37,5 +37,5 @@ func DefaultNamespaceOptions() (NamespaceOptions, error) {
 
 // getNetworkInterface creates the network interface
 func getNetworkInterface(store storage.Store, cniConfDir, cniPluginPath string) (nettypes.ContainerNetwork, error) {
-	return nil, errors.New("function not supported on non-linux systems")
+	return nil, nil
 }