FR: linux: open login URL during "tailscale login" using $BROWSER

### What are you trying to do?

Logging in to tailscale on a Linux machine using "sudo tailscale login" prints a login URL but requires the user to open it in the browser manually.

### How should we solve this?

Open a tab with the login URL automatically on `tailscale login`, using `$BROWSER` or some other standard-ish Linux mechanism. Maybe lookup common browser binary names in `$PATH` as a fallback.

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

Minor onboarding friction, you have to click the URL if your terminal supports this or copy-paste it into your browser.

### Anything else?

OS: EndeavourOS (Arch-based).
Tailscale version: 1.44.0-dev20230621
Browser: firefox