jenkins: move $out/{lib => webapps}/jenkins.war

As pointed out by @danbst, the tomcat NixOS module expects packages
listed in services.tomcat.webapps to either be direct .war file paths or
have .war files inside a "webapps" directory.

Commit 4075c10a
("jenkins: move .war file from $out to $out/lib/jenkins.war") broke
jenkins + tomcat. Fix it by moving jenkins.war to $out/webapps/.