Update to newest compiler with new value for Async flag

Author: agocke

src/tests/Directory.Build.props

@@ -10,7 +10,7 @@
 
   <PropertyGroup>
     <!-- Override the compiler version with a private build that supports async2 -->
-    <MicrosoftNetCompilersToolsetVersion>4.14.0-3.25170.6</MicrosoftNetCompilersToolsetVersion>
+    <MicrosoftNetCompilersToolsetVersion>5.0.0-1.25222.4</MicrosoftNetCompilersToolsetVersion>
   </PropertyGroup>
 
   <PropertyGroup>