k8s worker: Set $USER explicitly
k8s worker: Set $USER explicitly

So few variables are set when `user_data` is run

fixes, from `/var/log/cloud-init-output.log`
```
main: line 157: USER: unbound variable
```

cunnie/bin,cunnie/bin,cunnie/bin,cunnie/bin,cunnie/bin,cunnie/bin,cunnie/bin