Fix variable name in ssh-exec.php

Summary: Fixes T9176. This was renamed in D13589 but a use site was missed.

Test Plan:
Before:

```
epriestley@orbital ~/dev/phabricator $ SSH_ORIGINAL_COMMAND=conduit sudo -E ./bin/ssh-exec --phabricator-ssh-user admin
phabricator-ssh-exec: Invalid command.
```

After:

```
epriestley@orbital ~/dev/phabricator $ SSH_ORIGINAL_COMMAND=conduit sudo -E ./bin/ssh-exec --phabricator-ssh-user admin
phabricator-ssh-exec: No Conduit method provided.
```

Reviewers: chad, joshuaspence

Reviewed By: chad, joshuaspence

Maniphest Tasks: T9176

Differential Revision: https://secure.phabricator.com/D13893

scripts/ssh/ssh-exec.php

@@ -230,7 +230,7 @@ try {
 
   $parsed_args = new PhutilArgumentParser($parseable_argv);
 
-  if (empty($workflow_names[$command])) {
+  if (empty($workflows[$command])) {
     throw new Exception(pht('Invalid command.'));
   }