patch 8.2.1126: Vim9: using :copen causes an error

Problem:    Vim9: using :copen causes an error.
Solution:   Add flag LET_NO_COMMAND in set_var().
diff --git a/src/evalvars.c b/src/evalvars.c
index 1314867..3dab22b 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -2852,7 +2852,7 @@
     typval_T	*tv,
     int		copy)	    // make copy of value in "tv"
 {
-    set_var_const(name, NULL, tv, copy, 0);
+    set_var_const(name, NULL, tv, copy, LET_NO_COMMAND);
 }
 
 /*