[feat] Add support for custom WiX toolkit installation URL

[dstaley]

Describe the problem

The current version of WiX that's automatically downloaded doesn't allow for the creation of installers that work on ARM64 Windows devices. That functionality was introduced in a development build of WiX.

Describe the solution you'd like

Currently the URL used to download WiX is hardcoded, but it would be nice if Tauri also checked for the presence of a WIX_TOOLKIT_URL environment variable that took precedence over the built-in value. This would allow users who want an ARM64 installer to manually configure their build with a version of WiX that supports ARM64.

Alternatives considered

I'm aware that v2 is moving to NSIS in lieu of WiX, but I don't expect that to be backported to v1 (or available in the immediate future). Also I don't think Tauri should default to the v3.14 version since as I understand it, it introduced breaking changes. Speaking of, I'm also assuming that upgrading to WiX v4 is a significant undertaking, so that's not an option either.

Additional context

I'd be more than happy to put this PR together. I just wanted to file an issue first so I could solicit feedback!

[FabianLars]

Not answering on the actual request, only the Alternatives section:

I'm aware that v2 is moving to NSIS in lieu of WiX, but I don't expect that to be backported to v1

nsis will be part of v1.3 and it's already merged in dev.

Also I don't think Tauri should default to the v3.14 version since as I understand it, it introduced breaking changes.

yeah quite a few breaking changes, it's closer to v4 than to v3. The last time i tried it a few months ago it also felt a bit buggy but maybe i messed up 🤷 Either way, from testing i wasn't comfortable introducing it in tauri, especially because it looks it's not actually supposed to be used and only serves as a migration step to v4

I'm also assuming that upgrading to WiX v4 is a significant undertaking, so that's not an option either.

Probably yeah. i'd still like to at least look at it once there is a stable release.