nixos/jenkins: set StateDirectory if home is /var/lib/jenkins

This will cause systemd to chown /var/lib/jenkins to the user that
the jenkins systemd service runs as, fixing permission issues when
mounting /var/lib/jenkins from another data volume.

It uses the same logic that garage is using, too, checking for the
prefix.

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

@@ -236,6 +236,7 @@ in {
 
       serviceConfig = {
         User = cfg.user;
+        StateDirectory = mkIf (hasPrefix "/var/lib/jenkins" cfg.home) "jenkins";
       };
     };
   };