Fixes Steps container spec serialization/deserialization

When using `Task/ClusterTask` API with dynamic client, serlization/deserialization
adds container as child key as Spec.steps[*].Container. Ideally, it should preserver
the step spec state and should not add the step spec details under the child
key container

This patch fixes the issue by adding right json directive in step
spec.

Fixes #2150

pkg/apis/pipeline/v1beta1/task_types.go

@@ -103,7 +103,7 @@ type TaskResult struct {
 // Step embeds the Container type, which allows it to include fields not
 // provided by Container.
 type Step struct {
-	corev1.Container
+	corev1.Container `json:",inline"`
 
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