Set flex only when using the default width

This was necessary to override the behavior of adding
flex element to the style. The spinner (activity indicator)
needs to have this set to false.

core/src/toga/style/pack.py

@@ -764,9 +764,10 @@ def __css__(self):
         # direction
         css.append(f"flex-direction: {self.direction.lower()};")
         # flex
-        if (self.width == NONE and self.direction == ROW) or (
-            self.height == NONE and self.direction == COLUMN
-        ):
+        if (
+            (self.width == NONE and self.direction == ROW)
+            or (self.height == NONE and self.direction == COLUMN)
+        ) and getattr(self, "_use_default_width", True):
             css.append(f"flex: {self.flex} 0 0;")
 
         # width/flex