nixos/tests/jenkins: increase disk size to 2 GiB

Or else Jenkins stops the built-in node due to lack of disk space (when
using the default 1 GiB).

nixos/tests/jenkins.nix

@@ -60,6 +60,11 @@ import ./make-test-python.nix ({ pkgs, ...} : {
         users.users.jenkins.extraGroups = [ "users" ];
 
         systemd.services.jenkins.serviceConfig.TimeoutStartSec = "6min";
+
+        # Increase disk space to prevent this issue:
+        #
+        # WARNING h.n.DiskSpaceMonitorDescriptor#markNodeOfflineOrOnline: Making Built-In Node offline temporarily due to the lack of disk space
+        virtualisation.diskSize = 2 * 1024;
       };
 
     slave =