Fix status.lastRegistrationCheckTime in body must be of type string: \"null\" error

[mumoshu]

Follow-up for #392

[int128]

FYI: I have deployed 9b87156 and got the following error:

2021-03-18T00:19:02.734Z	ERROR	controllers.Runner	Failed to update runner status	{"runner": "NAME", "error": "Runner.actions.summerwind.dev \"NAME\" is invalid: status.lastRegistrationCheckTime: Invalid value: \"null\": status.lastRegistrationCheckTime in body must be of type string: \"null\""}
...
github.com/summerwind/actions-runner-controller/controllers.(*RunnerReconciler).updateRegistrationToken
	/workspace/controllers/runner_controller.go:467

I hope this will fix it.

[mumoshu]

Turns out this happens when the registration token is being renewed here

https://github.com/summerwind/actions-runner-controller/blob/7a7086e7aa14848ec19fe3a9ed6b1f97277397f2/controllers/runner_controller.go#L466

This in conjunction with the wrong CRD scheme resulted in the error. In other places those errors doesn't occur as we use Patch instead of Update there.