fix(maker-wix): set default arch

[ShGKme]

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Summarize your changes:

electron-wix-msi defaults to x86 arch.

Even if @electron-forge is set to make x64 installer, by default x86 installer will be created, and the app will be installed to C:\Program Files (x86)\.

⚠️ Note: this might be considered a breaking change - new installation will be in a new directory (C:\Program Files\ after C:\Program Files (x86)\).

Alternative: maker users can manually pass arch.

[erickzhao]

This LGTM. I think you're right about this being a breaking change, but I think building according to the arch makes the most sense according to how we normally output Forge distributables.

I'm a +1 for this, but only merging after we start the Forge 8 release process.