Debug build failure?

The **Windows - .NET Core** > **Prepare Solution** step is
failing:

    ##[error]build-tools\VersionInfo\VersionInfo.csproj(0,0): Error MSB4236: The SDK 'Microsoft.Build.NoTargets' specified could not be found.
    D:\a\1\s\build-tools\VersionInfo\VersionInfo.csproj : error MSB4236: The SDK 'Microsoft.Build.NoTargets' specified could not be found.

…which doesn't make sense, as `Microsoft.Build.NoTargets` is
versioned in `global.json`, and similar usage works in
xamarin-android…

Hypothesis: it works elsewhere because the `.sln` is in the same
dir as the `global.json`, which isn't the case here.

Theory: adding `build-tools/global.json` will fix things.

Time to test!

Author: jonpryor

build-tools/global.json

@@ -0,0 +1,8 @@
+{
+    "sdk": {
+        "allowPrerelease": true
+    },
+    "msbuild-sdks": {
+        "Microsoft.Build.NoTargets": "2.0.1"
+    }
+}