scripts: fix bin/cqlsh shortcut

Since 3c7af28725, the cqlsh submodule no longer contains a
bin/cqlsh shell script. This broke the supermodule's bin/cqlsh
shortcut.

Fix it by invoking cqlsh.py directly.

Closes scylladb/scylladb#20591

bin/cqlsh

@@ -4,5 +4,5 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 here=$(dirname "$0")
-exec "$here/../tools/cqlsh/bin/cqlsh" "$@"
+exec "$here/../tools/cqlsh/bin/cqlsh.py" "$@"