[CP to release/1.5-stable] Fix centrally-versioned package references

[JesseCol]

Cherry pick from https://github.com/microsoft/WindowsAppSDK/pull/4674

For the projects under the VSIX folder, the build stopped honoring the "Directory.Packages.props" file at the root of the repo. This causes us to pick up old versions of all these packages, and hilarity ensues.

I'm not sure at the moment what caused this change in behavior, but this fix looks safe as we continue to investigate.

[JesseCol]

/azp run

[azure-pipelines[bot]]

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