Appveyor CI failing due to nuget restore

[mikkelbu]

Builds are failing on AppVeyor in nuget restore, see e.g. #549 (comment).

...
Installing NUnit 3.11.0.
  OK https://ci.appveyor.com/nuget/nunit/FindPackagesById()?id='System.Reflection.TypeExtensions'&semVerLevel=2.0.0 472ms
NU3005: Package 'Microsoft.NETCore.Platforms 2.0.0' from source 'C:\Program Files\dotnet\sdk\NuGetFallbackFolder': The package signature file entry is invalid. The central directory header field 'compression method' has an invalid value (8).
  GET https://www.myget.org/F/nunit-analyzers/api/v3/flatcontainer/nunit/index.json
  NotFound https://www.myget.org/F/nunit-analyzers/api/v3/flatcontainer/nunit/index.json 194ms
...

The problem with nuget is the same as in #515 which should be solved by #525, but it seems like the solution is not enough.

AppVeyor had new images uploaded 11 February (see e.g. AppVeyor Windows images update. There was no changes to msbuild, but nuget was bumped, so nuget now probably calls to the same msbuild version that was failing in #515.

[rprouse]

I missed this before my comments on Charlie's PR. I see that your PR attempt hasn't worked @mikkelbu. Since an older version of NuGet.exe worked, could we include that older version and do the restore using that?

[mikkelbu]

@rprouse That would probably work, but I've found another solution which is even simpler (but it took a lot of attempts 😄).

[CharliePoole]

Nice!