perf: timeout while submitting the purchase receipt entry

(cherry picked from commit 1fa6233377)

erpnext/buying/doctype/purchase_order/purchase_order.py

@@ -457,6 +457,7 @@ class PurchaseOrder(BuyingController):
 			)
 
 
+@frappe.request_cache
 def item_last_purchase_rate(name, conversion_rate, item_code, conversion_factor=1.0):
 	"""get last purchase rate for an item"""