create-production-venv: Fix symlink creation.

The install script was failing on 2nd+ attempts if the first attempt
was interrupted.

This failure happened because zulip-venv already existed at
`current_venv_path`. Changing the `ln` command's flags from `-s` to
`-nsf` should make this part of the script idempotent.

scripts/lib/create-production-venv

@@ -32,5 +32,5 @@ cached_venv_path = setup_virtualenv(
     virtualenv_args=['-p', 'python{}'.format(python_version)])
 
 current_venv_path = os.path.join(args.deploy_path, 'zulip-current-venv')
-run(['ln', '-s', venv_name, current_venv_path])
+run(['ln', '-nsf', venv_name, current_venv_path])
 # Now the virtualenv has been activated