new toolset compiler - 2.6.0-beta1-62126-01 (#2090)

This was a "special" build, so there is no public VSIX for this.
The old one should be ok, for most scenarios.

When VS 15.5 preview1 is available it will match this compiler feature-wise by default.

Author: VSadov

Directory.Build.props

@@ -14,7 +14,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NETCore.Compilers" Version="2.6.0-rdonly-ref-62111-06" PrivateAssets="All">
+    <PackageReference Include="Microsoft.NETCore.Compilers" Version="2.6.0-beta1-62126-01" PrivateAssets="All">
       <!--
         Suppresses the warning about having an explicit package version in this repo.
         We are okay with this for now as the experimental version of the compiler is unique to Kestrel (for now).