FR: tailscale login should have --accept-risk=lose-ssh flag

### What are you trying to do?

`tailscale login` eagerly logs out. I wanted to switch accounts of a remote tailscale device and using `tailscale login --nickname newaccount` immediately logged me out AND broke the SSH connection.

### How should we solve this?

The logout can wait (maybe?) till the new account has authenticated. This might enable switching accounts on tailscale devices that are remote.

Even if we don't implement lazy logout, not losing SSH connection in this scenario will be a win.

### What is the impact of not solving this?

- It is not possible to switch tailscale account on remote machines (without using auth keys)
- It is easy to think that you can do it but login command doesn't stop you from getting locked out

### Anything else?

_No response_