feat: set constant tarEntry mtime to reproduce packages

Author: nefelim4ag

src/UnityNuGet/RegistryCache.cs

@@ -1062,6 +1062,7 @@ private static async Task WriteBufferToTar(TarOutputStream tarOut, string filePa
             filePath = filePath.TrimStart('/');
 
             var tarEntry = TarEntry.CreateTarEntry($"package/{filePath}");
+            tarEntry.ModTime = DateTime.UnixEpoch;
             tarEntry.Size = buffer.Length;
             await tarOut.PutNextEntryAsync(tarEntry, cancellationToken);
             await tarOut.WriteAsync(buffer, cancellationToken);