vs2015 runtime finds unusable SDK version

[mdekstrand]

With Visual Studio 2019 on Windows Server 2019 for GitHub Actions, the activation script for vs2015_win-64 sometimes tries to activate unusable SDK version wdf.

An example of this failure is in "Build Packages on windows" in this GitHub action: https://github.com/lenskit/lkpy/runs/639364270

[mdekstrand]

I suspect, but have not yet confirmed, that the problem arises when the Windows Driver Kit is installed.

[mdekstrand]

I have confirmed that the Windows Driver Kit, which is included in GitHub Actions images, is responsible for creating the problematic SDK version.