Switch vcpkg cache for Windows build check to NuGet

[kblaschke]

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Both vcpkg and GitHub removed support for GitHub Actions Cache, so the current best way of caching packages on Windows is via NuGet.

Uses a classic PAT in my account to access the package repository. The token is added as a secret in the projectM org.

Sadly, passing secrets to jobs triggered by a PR from a forked repository (like this one here) doesn't work, so these build checks will still take ~15mins. Push and post-merge builds will run faster though.