dotfiles: pin WiX to 5.0.2, the last release that owes no fee

winget install WiXToolset.WiXCLI has no version selector, so it installed
7.0.0. From 6.0 onward the WiX package carries OSMFEULA.txt on top of the
Microsoft Reciprocal License: an Open Source Maintenance Fee agreement
charging a monthly fee to anyone using the prebuilt binaries as part of
revenue-generating activity with annual gross revenue >= US$10,000.

The fee buys the binaries; it is not a license fee and restricts nothing
about what we package. MS-RL is file-scoped and never reached the MSIs
WiX builds under any version. Pinning to 5.0.2 is about the invoice.

Install it as a .NET global tool instead, which takes a --version, and
add the .NET 10 SDK it needs. The MSBuild half still has to be pinned in
each .wixproj; the comment says so where someone will look for it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012gCzbrN6p3emFUyufuJsyq