Tailscale install script won't work if restarted after adding repository on OpenSUSE

### What is the issue?

If the Tailscale install script is interrupted or fails mid-install on OpenSUSE, rerunning the script fails. I encountered this because I had a bad repository in my settings. The install script added the Tailscale repo, then crashed due to the bad repository. I removed the bad repository and restarted the script, but the script fails because the Tailscale repo already exists.

### Steps to reproduce

1. Add a bad repository (or interrupt the install script during the next step)
2. Run `curl -fsSL https://tailscale.com/install.sh | sh`, the installer will fail after adding the Tailscale repo.
3. Remove the bad repository.
4. Rerun the command. You'll get this output:

```
$ curl -fsSL https://tailscale.com/install.sh | sh
Installing Tailscale for opensuse tumbleweed, using method zypper
+ sudo rpm --import https://pkgs.tailscale.com/stable/opensuse/tumbleweed/repo.gpg
+ sudo zypper --non-interactive ar -g -r https://pkgs.tailscale.com/stable/opensuse/tumbleweed/tailscale.repo
Adding repository 'Tailscale stable' ..................................................................[error]
Repository named 'tailscale-stable' already exists. Please use another alias.
```

### Are there any recent changes that introduced the issue?

_No response_

### OS

Linux

### OS version

OpenSUSE Tumbleweed

### Tailscale version

N/A

### Other software

_No response_

### Bug report

_No response_