fix: hcm caret shouldn't be allowed to shrink (#1733)

Caret shouldn't shrink if component is truncating. Component only used
in enterprise right now, and there's a duplicate change in
deephaven-ent/iris#1274

Author: dsmmcken

packages/components/src/HierarchicalCheckboxMenu.scss

@@ -18,6 +18,7 @@
     @include caret-icon;
 
     margin-right: 4px;
+    flex-shrink: 0;
   }
 }