`skaffold dev` watch mode cannot be used with k8s Jobs.

### Expected behavior
when I am iteratively developing an app with a `Job`, I can update the `Job` more than once and it will run successfully.

### Actual behavior
`skaffold dev` always throws error on the second iteration:
```
- The Job "python-guestbook-migration" is invalid: spec.template: Invalid value: core.PodTemplateSpec{ObjectMeta:v1.ObjectMeta{Name:"python-guestbook-migration", GenerateName:"", Namespace:"", SelfLink:"", UID:"", ResourceVersion:"", Generation:0, CreationTimestamp:v1.Time{Time:time.Time{wall:0x0, ext:0, loc:(*time.Location)(nil)}}, DeletionTimestamp:(*v1.Time)(nil), DeletionGracePeriodSeconds:(*int64)(nil), Labels:map[string]string{"app.kubernetes.io/managed-by":"skaffold", "controller-uid":"87bc387d-47d9-4598-a3ca-3737657222dd", "job-name":"python-guestbook-migration", "skaffold.dev/run-id":"33c89cb7-be13-4868-81ac-89a96c7004f4"}, Annotations:map[string]string(nil), OwnerReferences:[]v1.OwnerReference(nil), Finalizers:[]string(nil), ClusterName:"", ManagedFields:[]v1.ManagedFieldsEntry(nil)}, Spec:core.PodSpec{Volumes:[]core.Volume(nil), InitContainers:[]core.Container{core.Container{Name:"check-db-ready", Image:"postgres", Command:[]string{"sh", "-c", "until pg_isready -h python-guestbook-db -p 5432; do echo waiting for database; sleep 2; done;"}, Args:[]string(nil), WorkingDir:"", Ports:[]core.ContainerPort(nil), EnvFrom:[]core.EnvFromSource(nil), Env:[]core.EnvVar(nil), Resources:core.ResourceRequirements{Limits:core.ResourceList(nil), Requests:core.ResourceList(nil)}, VolumeMounts:[]core.VolumeMount(nil), VolumeDevices:[]core.VolumeDevice(nil), LivenessProbe:(*core.Probe)(nil), ReadinessProbe:(*core.Probe)(nil), StartupProbe:(*core.Probe)(nil), Lifecycle:(*core.Lifecycle)(nil), TerminationMessagePath:"/dev/termination-log", TerminationMessagePolicy:"File", ImagePullPolicy:"Always", SecurityContext:(*core.SecurityContext)(nil), Stdin:false, StdinOnce:false, TTY:false}}, Containers:[]core.Container{core.Container{Name:"python-guestbook-migration", Image:"python-guestbook-frontend-django:bd8e4ace19574f76515a2784deb306e3aceaa94f4c6c2030ef1f02df47971eec", Command:[]string{"/bin/sh", "-c", "python manage.py migrate"}, Args:[]string(nil), WorkingDir:"", Ports:[]core.ContainerPort(nil), EnvFrom:[]core.EnvFromSource(nil), Env:[]core.EnvVar{core.EnvVar{Name:"DATABASE_NAME", Value:"postgres", ValueFrom:(*core.EnvVarSource)(nil)}, core.EnvVar{Name:"DATABASE_USER", Value:"postgres", ValueFrom:(*core.EnvVarSource)(nil)}, core.EnvVar{Name:"DATABASE_HOST", Value:"python-guestbook-db", ValueFrom:(*core.EnvVarSource)(nil)}, core.EnvVar{Name:"DATABASE_PORT", Value:"5432", ValueFrom:(*core.EnvVarSource)(nil)}}, Resources:core.ResourceRequirements{Limits:core.ResourceList(nil), Requests:core.ResourceList(nil)}, VolumeMounts:[]core.VolumeMount(nil), VolumeDevices:[]core.VolumeDevice(nil), LivenessProbe:(*core.Probe)(nil), ReadinessProbe:(*core.Probe)(nil), StartupProbe:(*core.Probe)(nil), Lifecycle:(*core.Lifecycle)(nil), TerminationMessagePath:"/dev/termination-log", TerminationMessagePolicy:"File", ImagePullPolicy:"IfNotPresent", SecurityContext:(*core.SecurityContext)(nil), Stdin:false, StdinOnce:false, TTY:false}}, EphemeralContainers:[]core.EphemeralContainer(nil), RestartPolicy:"OnFailure", TerminationGracePeriodSeconds:(*int64)(0xc00e938ed8), ActiveDeadlineSeconds:(*int64)(nil), DNSPolicy:"ClusterFirst", NodeSelector:map[string]string(nil), ServiceAccountName:"", AutomountServiceAccountToken:(*bool)(nil), NodeName:"", SecurityContext:(*core.PodSecurityContext)(0xc00a7ab280), ImagePullSecrets:[]core.LocalObjectReference(nil), Hostname:"", Subdomain:"", SetHostnameAsFQDN:(*bool)(nil), Affinity:(*core.Affinity)(nil), SchedulerName:"default-scheduler", Tolerations:[]core.Toleration(nil), HostAliases:[]core.HostAlias(nil), PriorityClassName:"", Priority:(*int32)(nil), PreemptionPolicy:(*core.PreemptionPolicy)(nil), DNSConfig:(*core.PodDNSConfig)(nil), ReadinessGates:[]core.PodReadinessGate(nil), RuntimeClassName:(*string)(nil), Overhead:core.ResourceList(nil), EnableServiceLinks:(*bool)(nil), TopologySpreadConstraints:[]core.TopologySpreadConstraint(nil)}}: field is immutable
```

### Information

- Skaffold version: 1.20.0
- Operating system: macOS 11.2.1
- Installed via: Cloud Code

### Steps to reproduce the behavior

1. clone the [Cloud Code Django Guestbook Sample](https://github.com/GoogleCloudPlatform/cloud-code-samples/tree/master/python/django/python-guestbook).
2. run `skaffold dev` ensure watch mode is enabled.
3. Update any file in the Django app.
4. Observe the error.