patch 8.2.4682: Vim9: can use :unlockvar for const variable

Problem:    Vim9: can use :unlockvar for const variable. (Ernie Rael)
Solution:   Check whether the variable is a const.
diff --git a/src/userfunc.c b/src/userfunc.c
index 052923e..c80a490 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1697,7 +1697,7 @@
 	{
 	    if (!did_type && type != NULL && ht == get_script_local_ht())
 	    {
-		svar_T  *sv = find_typval_in_script(tv, 0);
+		svar_T  *sv = find_typval_in_script(tv, 0, TRUE);
 
 		if (sv != NULL)
 		    *type = sv->sv_type;