hydra: Fix typo.

* build-aux/hydra/gnu-system.scm (hydra-jobs): Use `cons', not `append'.

build-aux/hydra/gnu-system.scm

@@ -130,8 +130,8 @@ (define (cross-jobs system)
                   (fold-packages (lambda (package result)
                                    (if (member package base-packages)
                                        result
-                                       (append (package-job store (job-name package)
-                                                            package system)
-                                               result)))
+                                       (cons (package-job store (job-name package)
+                                                          package system)
+                                             result)))
                                  (cross-jobs system)))
                 systems)))