Fix an issue where SSH workflows would execute immediately

Summary: rPe397103bf broke this since one of the implementations was `return false;`, not `return true;`.

Auditors: chad

src/infrastructure/ssh/PhabricatorSSHWorkflow.php

@@ -6,6 +6,10 @@ abstract class PhabricatorSSHWorkflow extends PhabricatorManagementWorkflow {
   private $iochannel;
   private $errorChannel;
 
+  public function isExecutable() {
+    return false;
+  }
+
   public function setErrorChannel(PhutilChannel $error_channel) {
     $this->errorChannel = $error_channel;
     return $this;