[IslandWindow.cpp] Avoid double-fetching dropdownDuration (#11383)

## Summary of the Pull Request
The code saved `args.DropdownDuration()` to a local and then called the function again, instead of using the local.
Changed to use the local.

## PR Checklist
* [ ] Closes #xxx
* [x] CLA signed
* [ ] Tests added/passed
* [ ] Documentation updated
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.

## Detailed Description of the Pull Request / Additional comments

I think this getter simply accesses a member on `args`, it doesn't parse the settings or anything, so compiler optimizes it, but seemed to make more sense to use the local.

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -1191,7 +1191,7 @@ void IslandWindow::_summonWindowRoutineBody(Remoting::SummonWindowBehavior args)
 {
     uint32_t actualDropdownDuration = args.DropdownDuration();
     // If the user requested an animation, let's check if animations are enabled in the OS.
-    if (args.DropdownDuration() > 0)
+    if (actualDropdownDuration > 0)
     {
         BOOL animationsEnabled = TRUE;
         SystemParametersInfoW(SPI_GETCLIENTAREAANIMATION, 0, &animationsEnabled, 0);