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().

src/evalvars.c

@@ -2852,7 +2852,7 @@ set_var(
     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);
 }
 
 /*

src/testdir/test_vim9_script.vim

@@ -2219,6 +2219,12 @@ def Test_source_vim9_from_legacy()
   delete('Xvim9_script.vim')
 enddef
 
+def Test_vim9_copen()
+  # this was giving an error for setting w:quickfix_title
+  copen
+  quit
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1126,
 /**/
     1125,
 /**/