ctr/content: make editor flag is required
ctr/content: make editor flag is required

The $EDITOR is not clear for user. If the env doesn't set the value for
$EDITOR, the users don't know how to use this subcommand when they get
error like this:

```
sh: 1: /tmp/edit-605573012: Permission denied
```

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

dmcgowan/containerd,dmcgowan/containerd,thaJeztah/containerd,thaJeztah/containerd,miaoyq/containerd,estesp/containerd,containerd/containerd,crosbymichael/containerd,containerd/containerd,mikebrow/containerd,crosbymichael/containerd,mikebrow/containerd,vdemeester/containerd,vdemeester/containerd,miaoyq/containerd,estesp/containerd