operating-systems: buer: Use quotes for lists which content is atoms-only

operating-systems/buer.scm

@@ -211,13 +211,17 @@
                             (list channel:guix
                                   channel:radix)))
                     (substitute-urls
-                      (list "https://ci.guix.gnu.org"
+                      `("https://ci.guix.gnu.org"
                         "https://berlin.guix.gnu.org"
                         "https://mirrors.sjtug.sjtu.edu.cn/guix"
                         "https://hydra-guix-129.guix.gnu.org"
                         "https://bordeaux-singapore-mirror.cbaines.net"
                         "https://bordeaux-us-east-mirror.cbaines.net"))
-                    (extra-options `("--max-jobs=8" "--cores=4"))))
+                    (extra-options
+                      `("--max-jobs=8"
+                        "--cores=4"
+                        "--gc-keep-derivations=yes"
+                        "--gc-keep-outputs=yes"))))
           (service guix-publish-service-type
                    (guix-publish-configuration
                     (port 8080)