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 4075c10a59
("jenkins: move .war file from $out to $out/lib/jenkins.war") broke
jenkins + tomcat. Fix it by moving jenkins.war to $out/webapps/.

nixos/modules/services/continuous-integration/jenkins/default.nix

@@ -154,7 +154,7 @@ in {
       '';
 
       script = ''
-        ${pkgs.jdk}/bin/java -jar ${pkgs.jenkins}/lib/jenkins.war --httpListenAddress=${cfg.listenAddress} \
+        ${pkgs.jdk}/bin/java -jar ${pkgs.jenkins}/webapps/jenkins.war --httpListenAddress=${cfg.listenAddress} \
                                                   --httpPort=${toString cfg.port} \
                                                   --prefix=${cfg.prefix} \
                                                   ${concatStringsSep " " cfg.extraOptions}