build right proto on unix

Author: dsyme

proto.proj

@@ -8,7 +8,7 @@
   <ItemGroup>
     <Projects Include="src\fsharp\FSharp.Build\FSharp.Build.fsproj">
       <AdditionalProperties Condition="'$(OS)' != 'Unix'">TargetFramework=net46</AdditionalProperties>
-      <AdditionalProperties Condition="'$(OS)' == 'Unix'">TargetFramework=netcoreapp2.1</AdditionalProperties>
+      <AdditionalProperties Condition="'$(OS)' == 'Unix'">TargetFramework=netstandard2.0</AdditionalProperties>
     </Projects>
     <Projects Include="src\fsharp\fsc\fsc.fsproj">
       <AdditionalProperties Condition="'$(OS)' != 'Unix'">TargetFramework=net46</AdditionalProperties>