Set version for ILMerge to 6.4.1-zlocal.1

If you don't set it specifically, after ILMerge the NuGet libraries
will show up as having Chocolatey's version. This affects the user
agent, and other uses of the library version.

Brought forward from this commit:
chocolatey/nuget-chocolatey@2a0aa36

Co-Authored-By: Rob Reynolds <63502+ferventcoder@users.noreply.github.com>

src/NuGet.Core/NuGet.Common/ClientVersionUtility.cs

@@ -9,7 +9,7 @@ namespace NuGet.Common
     public static class ClientVersionUtility
     {
         // Cache the value since it cannot change
-        private static string _clientVersion;
+        private static string _clientVersion = "6.4.1-zlocal.1";
 
         /// <summary>
         /// Find the current NuGet client version from the assembly info as a string.