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 4075c10a59af455db2012beb313453662f09dad0
("jenkins: move .war file from $out to $out/lib/jenkins.war") broke
jenkins + tomcat. Fix it by moving jenkins.war to $out/webapps/.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.