Turn off parallel package restore by default

It is currently causing intermittent restore failure

Author: nguerrera

dir.props

@@ -85,7 +85,7 @@
 
     <DnuRestoreCommand>"$(DnuToolPath)"</DnuRestoreCommand>
     <DnuRestoreCommand>$(DnuRestoreCommand) restore</DnuRestoreCommand>
-    <DnuRestoreCommand>$(DnuRestoreCommand) --parallel</DnuRestoreCommand>
+    <DnuRestoreCommand Condition="'$(ParallelRestore)'=='true'">$(DnuRestoreCommand) --parallel</DnuRestoreCommand>
     <DnuRestoreCommand>$(DnuRestoreCommand) --packages "$(PackagesDir.TrimEnd('/\'.ToCharArray()))" $(DnuRestoreSource)</DnuRestoreCommand>
     <DnuRestoreCommand Condition="'$(LockDependencies)' == 'true'">$(DnuRestoreCommand) --lock</DnuRestoreCommand>
   </PropertyGroup>