RoslynTargetsPath appears to be not set for VisualStudio - unity3d

Question

Today (without me initiating any updates) VisualStudio (VisualStudio for Mac Community Version 8.0.6 build 4) all of a sudden kept telling me I needed to upgrade Mono (I'm on a Mac). I did so. After that none of my projects would compile because it seems the configuration is messed up and I think RoslynTargetsPath is not set. I'm using VisualStudio with Unity if that makes any difference.
The first error I got was this
"~/Library/Caches/VisualStudio/8.0/MSBuild/17745_1//Microsoft.CSharp.Core.targets
was not found"
Looking at the file system I saw that the file was in the Rosyln subfolder. So I edited my Assembly-CSharp.csproj file and changed the import definition from
<Import Project="$(MSBuildToolsPath)/Microsoft.CSharp.Core.targets" />
to
<Import Project="$(MSBuildToolsPath)/Roslyn/Microsoft.CSharp.Core.targets" />
Once I did this the error changed to
"The target "Build" does not exist in the project. (MSB4057)"
I researched that and tried adding this to the project file
The error went back to this
"~/Library/Caches/VisualStudio/8.0/MSBuild/17745_1//Microsoft.CSharp.Core.targets
was not found".
Notice the "//" in the path. I checked my MSBuild.dll.config and the definition for RoslynTargetsPath is this
<property name="RoslynTargetsPath" value="$([MSBuild]::GetToolsDirectory32())\Roslyn" />
So then I changed the import in my project file to this
<Import Project="$(RoslynTargetsPath)/Microsoft.CSharp.Core.targets" />
And the error changed to
"/Microsoft.CSharp.Core.targets was not found".
So it looks like RoslynTargetsPath is an empty string. I don't know why I was forced to upgrade Mono when VisualStudio had not been updated.