Fix back button sometimes not working after exiting a thread (#6732)

When I reproduced the issue, it looked like the coroutine used to mark as read and then exit the room was canceled, leaving the `markingAsReadAndExiting` variable with `true` value and preventing the exit block from running again.

features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/MessagesPresenter.kt

@@ -272,6 +272,8 @@ class MessagesPresenter(
                             }
                         }
                         navigator.close()
+                    }.invokeOnCompletion {
+                        markingAsReadAndExiting.set(false)
                     }
                 }
             }