Use read-only mode for sqlite3 completion

The fix from the previous commit *should* be enough to keep my
completion from creating `':memory:'` files, but belt and suspenders, kids.

`immutable=1` isn't enough - AFAICT, that just means "don't create
WAL/SHM files".  You need `mode=ro` to prevent SQLite from creating
non-existent database files, even for read-only operations.

Author: hoelzro

Completion/_sqlite

@@ -54,7 +54,7 @@ function _complete_sqlite() {
     fi
 
     db_file=(${~db_file})
-    db_file="file:$db_file[1]?immutable=1"
+    db_file="file:$db_file[1]?immutable=1&mode=ro"
 
     compset -q
     local -a sql_words