make arc install-certificate <uri> work if uri is specified

Summary: see title

Test Plan: ran arc install-certificate <uri> in a directory without an .arcconfig and it worked! ran arc install-certificate in a directory with an .arcconfig and it worked! ran arc install-certificate <uri> in a directory with an .arcconfig and noted it correctly overrode the .arcconfig

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2251

Differential Revision: https://secure.phabricator.com/D4332

src/workflow/ArcanistInstallCertificateWorkflow.php

@@ -52,6 +52,7 @@ EOTEXT
   public function run() {
 
     $uri = $this->determineConduitURI();
+    $this->setConduitURI($uri);
 
     echo "Installing certificate for '{$uri}'...\n";