installer: Ensure 'packages' field is a superset of '%base-packages'.

Fixes <https://bugs.gnu.org/35541>.
Reported by Florian Pelz <pelzflorian@pelzflorian.de>.

* gnu/installer/services.scm (system-services->configuration): Generate
a snippet that appends PACKAGES to %BASE-PACKAGES.

gnu/installer/services.scm

@@ -130,11 +130,13 @@ (define (system-services->configuration services)
     (if (null? snippets)
         `(,@(if (null? packages)
                 '()
-                `((packages (list ,@packages))))
+                `((packages (append (list ,@packages)
+                                    %base-packages))))
           (services ,base))
         `(,@(if (null? packages)
                 '()
-                `((packages (list ,@packages))))
+                `((packages (append (list ,@packages)
+                                    %base-packages))))
           (services (append (list ,@snippets
 
                                   ,@(if desktop?