Zui Insiders update notification should point to GitHub Releases

[philrz]

A community user pointed out in a Slack thread:

I'm using a linux desktop these days and when zui tells me about an update it takes me to this page: https://www.brimdata.io/download/ - should it take me to the zui-insiders releases page instead?

Indeed, I expect it would not be too difficult to put some logic here to offer an alternate URL if the user is running Zui Insiders.

https://github.com/brimdata/brim/blob/b7ee21a85a867f35332a547fda24053e3bc30c44/src/js/electron/autoUpdater.ts#L47-L51