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/.

pkgs/development/tools/continuous-integration/jenkins/default.nix

@@ -10,8 +10,8 @@ stdenv.mkDerivation rec {
   };
 
   buildCommand = ''
-    mkdir -p "$out/lib"
-    cp "$src" "$out/lib/jenkins.war"
+    mkdir -p "$out/webapps"
+    cp "$src" "$out/webapps/jenkins.war"
   '';
 
   meta = with stdenv.lib; {