Fixed: custom script error when importing some downloads

Co-authored-by: Qstick <376117+Qstick@users.noreply.github.com>

(cherry picked from commit 8f482c534f15c14a9b3097313a4f5e9273549d88)

Closes #2457

src/NzbDrone.Common/Processes/ProcessProvider.cs

@@ -131,14 +131,7 @@ namespace NzbDrone.Common.Processes
                         var key = environmentVariable.Key.ToString();
                         var value = environmentVariable.Value?.ToString();
 
-                        if (startInfo.EnvironmentVariables.ContainsKey(key))
-                        {
-                            startInfo.EnvironmentVariables[key] = value;
-                        }
-                        else
-                        {
-                            startInfo.EnvironmentVariables.Add(key, value);
-                        }
+                        startInfo.EnvironmentVariables[key] = value;
                     }
                     catch (Exception e)
                     {