Fetching from MASS uses the wrong script name

[jfrost-mo]

Describe the bug

fetch_fcst throws up a ~/cylc-run/cset-workflow/run7/bin/app_env_wrapper: line 61: exec: restricted-fetch-data-mass.sh: not found

This is because the script has been converted to python, and is now .py.

How to reproduce

Steps to reproduce the behaviour:

1. Run CSET while fetching from MASS
2. Observe failure.

Expected behaviour

The correct script is called from cset-workflow/app/fetch_fcst/opt/rose-app-mass.conf

Environment

• Version: [e.g. from cset --version]
• Browser (if UI issue): [e.g. Chrome, Firefox]