cache the result of _libgap_() in PermutationGroup

This fixes weird

sage: S = SymmetricGroup(4)
sage: g1 = S._libgap_()
sage: g2 = S._libgap_()
sage: g1.IsIdenticalObj(g2)
false

(one gets true now)

Author: dimpase

src/sage/groups/perm_gps/permgroup.py

@@ -684,7 +684,8 @@ def gap(self):
         """
         if self._libgap is not None:
             return self._libgap
-        return super()._libgap_()
+        self._libgap = super()._libgap_()
+        return self._libgap
 
     # Override the default _libgap_ to use the caching as self._libgap
     _libgap_ = gap