nixos/jenkins: add RuntimeDirectory

Necessary for jenkins socket which has to be enabled with

`JENKINS_UNIX_DOMAIN_PATH=/run/jenkins/jenkins.socket`

in `services.jenkins.environment`

https://www.github.com/jenkinsci/packaging/issues/455

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

@@ -237,6 +237,8 @@ in {
       serviceConfig = {
         User = cfg.user;
         StateDirectory = mkIf (hasPrefix "/var/lib/jenkins" cfg.home) "jenkins";
+        # For (possible) socket use
+        RuntimeDirectory = "jenkins";
       };
     };
   };