Fix a small memory leak in ExecReScanAgg() in the hashed aggregation case.

Fix a small memory leak in ExecReScanAgg() in the hashed aggregation case.
In the previous coding, the list of columns that needed to be hashed on
was allocated in the per-query context, but we reallocated every time
the Agg node was rescanned. Since this information doesn't change over
a rescan, just construct the list of columns once during ExecInitAgg().