Make dotnet cmd executable.

Author: shmao

external/runtime/runtime.depproj

@@ -73,6 +73,8 @@
           DestinationFolder="$(TestHostRootPath)"
           SkipUnchangedFiles="true"
           UseHardlinksIfPossible="true" />
+          
+    <Exec Command="chmod +x $(TestHostRootPath)dotnet" Condition="'$(RunningOnUnix)' == 'true'"/>
 
   </Target>