WIndows installer doesn't check for currently installed directory

### What is the issue?

When you are upgrading an existing Tailscale installation on Windows, the installer always tries to install to the default path, even when you have customised it.

**The installer should check the current installation path and use this as its default.** If a fresh install, installer can pick.

I have had this with another software developer. They contacted the dev of the installer package and were given instructions on how to solve this. I had a working version from them within 36 hours, so I would guess that this is not too difficult of a fix to implement.

### Steps to reproduce

1) Install and customise path during installation,
2) Install again.

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

No

### OS

Windows

### OS version

WIn10

### Tailscale version

1.78.1

### Other software

_No response_

### Bug report

_No response_