patch 9.0.0726: looping over list of lists works in script, not in function

Problem:    Looping over list of lists and changing the list contents works in
            Vim9 script, not in a compiled function.
Solution:   Mark the loop variable final instead of const. (closes #11347)

src/testdir/test_vim9_script.vim

@@ -2275,6 +2275,20 @@ def Test_for_loop()
   v9.CheckDefAndScriptSuccess(lines)
 enddef
 
+def Test_for_loop_list_of_lists()
+  # loop variable is final, not const
+  var lines =<< trim END
+      # Filter out all odd numbers in each sublist
+      var list: list<list<number>> = [[1], [1, 2], [1, 2, 3], [1, 2, 3, 4]]
+      for i in list
+          filter(i, (_, n: number): bool => n % 2 == 0)
+      endfor
+
+      assert_equal([[], [2], [2], [2, 4]], list)
+  END
+  v9.CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_for_loop_with_closure()
   # using the loop variable in a closure results in the last used value
   var lines =<< trim END

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    726,
 /**/
     725,
 /**/

src/vim9cmds.c

@@ -1048,7 +1048,7 @@ compile_for(char_u *arg_start, cctx_T *cctx)
 			&& need_type_where(item_type, lhs_type, -1,
 					    where, cctx, FALSE, FALSE) == FAIL)
 		    goto failed;
-		var_lvar = reserve_local(cctx, arg, varlen, ASSIGN_CONST,
+		var_lvar = reserve_local(cctx, arg, varlen, ASSIGN_FINAL,
 								     lhs_type);
 		if (var_lvar == NULL)
 		    // out of memory or used as an argument