It looks like on our machine the root is in /var/cache.

  % ps axfww
  [...]
  25835 ?        Sl    33:23  \_ /usr/bin/java -Djava.awt.headless=true -jar /usr/share/jenkins/jenkins.war --webroot=/var/cache/jenkins/war --httpPort=8080 --ajp13Port=-1

Hopefully this will fix all the errors we're seeing:

   % tail ~/logs/jenkins-error.log
   2014/12/15 09:30:14 [crit] 12548#0: *105 stat() "/var/run/jenkins/war/ajaxExecutors" failed (13: Permission denied), client: 208.185.155.170, server: jenkins.khanacademy.org, request: "POST /ajaxExecutors HTTP/1.1", host: "jenkins.khanacademy.org", referrer: "http://jenkins.khanacademy.org/"

Auditors: chris

Test plan:
Will install on jenkins and restart nginx and check jenkins-error.log again.