Fixes: slack notification to multiple targets

If you configure the slack integration to send out multiple notifications during one transaction, only the first one is sent it.

The variable result, which stores the rendered template, is overriden after sending out the first message to slack.

(Line numbers can be off as I modified the file)
```
zammad-scheduler_1      | E, [2023-03-14T07:52:51.356285 zammad#1-112060] ERROR -- : no implicit conversion of Symbol into Integer (TypeError)
zammad-scheduler_1      | app/models/transaction/slack.rb:200:in `block in perform'
zammad-scheduler_1      | app/models/transaction/slack.rb:118:in `each'
zammad-scheduler_1      | app/models/transaction/slack.rb:118:in `perform'
zammad-scheduler_1      | lib/transaction_dispatcher.rb:61:in `execute_single_backend'
zammad-scheduler_1      | app/jobs/transaction_job.rb:25:in `block in perform'
zammad-scheduler_1      | app/jobs/transaction_job.rb:21:in `perform'
zammad-scheduler_1      | lib/background_services/service/process_delayed_jobs.rb:25:in `block (2 levels) in launch'
zammad-scheduler_1      | lib/background_services/service/process_delayed_jobs.rb:23:in `block in launch'
zammad-scheduler_1      | lib/background_services/service/process_delayed_jobs.rb:20:in `loop'
zammad-scheduler_1      | lib/background_services/service/process_delayed_jobs.rb:20:in `launch'
zammad-scheduler_1      | lib/background_services/service.rb:27:in `block in run'
zammad-scheduler_1      | lib/application_handle_info.rb:19:in `use'
zammad-scheduler_1      | lib/background_services/service.rb:33:in `block in run_in_service_context'
zammad-scheduler_1      | lib/background_services/service.rb:32:in `run_in_service_context'
zammad-scheduler_1      | lib/background_services/service.rb:26:in `run'
zammad-scheduler_1      | lib/background_services.rb:67:in `block in start_as_thread'
```

app/models/transaction/slack.rb

@@ -179,21 +179,21 @@ def perform
       )
       if local_config['expand']
         body = "#{result[:subject]}\n#{result[:body]}"
-        result = notifier.ping body
+        result_ping = notifier.ping body
       else
         attachment = {
           text:      result[:body],
           mrkdwn_in: ['text'],
           color:     color,
         }
-        result = notifier.ping result[:subject],
+        result_ping = notifier.ping result[:subject],
                                attachments: [attachment]
       end
-      if !result.empty? && !result[0].success?
+      if !result_ping.empty? && !result_ping[0].success?
         if sent_value
           Rails.cache.delete(cache_key)
         end
-        Rails.logger.error "Unable to post webhook: #{local_config['webhook']}: #{result.inspect}"
+        Rails.logger.error "Unable to post webhook: #{local_config['webhook']}: #{result_ping.inspect}"
         next
       end
       Rails.logger.debug { "sent webhook (#{@item[:type]}/#{ticket.id}/#{local_config['webhook']})" }