UI: Fix cancel plugin command (#3903)

* UI: Fix cancel plugin command

As installing a plugin also adds the delete/uninstall command, we just select the last action, which is the one the user triggered. 

Fixes #3890.

* Apply suggestions from code review

https://stackoverflow.com/questions/50100688/check-if-value-tuple-is-default

BTCPayServer/Controllers/UIServerController.Plugins.cs

@@ -66,6 +66,7 @@ namespace BTCPayServer.Controllers
 
             return RedirectToAction("ListPlugins");
         }
+        
         [HttpPost("server/plugins/cancel")]
         public IActionResult CancelPluginCommands(
             [FromServices] PluginService pluginService, string plugin)
@@ -93,7 +94,6 @@ namespace BTCPayServer.Controllers
                 }
                 else
                 {
-
                     pluginService.InstallPlugin(plugin);
                 }
                 TempData.SetStatusMessageModel(new StatusMessageModel()