Use Tools from tools diirectory

src/FSharpSource.targets

@@ -798,7 +798,7 @@
     <NuGetPackagesPath Condition="'$(NuGetPackagesPath)' == ''">$(FSharpSourcesRoot)\..\packages</NuGetPackagesPath>
 
     <!-- fix this to be not so unfortunate looking -->
-    <BuildToolsDir>$(MSBuildThisFileDirectory)..\packages\Microsoft.DotNet.BuildTools.1.0.25-prerelease-00140\lib\</BuildToolsDir>
+    <BuildToolsDir>$(MSBuildThisFileDirectory)..\Tools\</BuildToolsDir>
 
     <!-- Implicitly needed by packageresolve.targets.  Should file a bug for a better error message here -->
     <PackagesDir>$(NuGetPackagesPath)\</PackagesDir>