Fix ordering of charges in CHARGE section when using to_prmtop

The ordering of the atomic charges in the CHARGE section of the
.prmtop was wrong because we iterated over a dictionary. Now we
excplicitly iterate over the atoms of interchange.topolgy to avoid
this.

openff/interchange/interop/amber/export/_export.py

@@ -515,8 +515,8 @@ def to_prmtop(interchange: "Interchange", file_path: Path | str):
 
         prmtop.write("%FLAG CHARGE\n" "%FORMAT(5E16.8)\n")
         charges = [
-            charge.m_as(unit.e) * AMBER_COULOMBS_CONSTANT
-            for charge in interchange["Electrostatics"].charges.values()
+            atom.partial_charge.m_as(unit.e) * AMBER_COULOMBS_CONSTANT
+            for atom in interchange.topology.atoms
         ]
         text_blob = "".join([f"{val:16.8E}" for val in charges])
         _write_text_blob(prmtop, text_blob)