R1:d22930c0fd6d

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…