ci: Support packages with multiple channels.

This is a follow-up of 61a1165340. For packages
provided by external channels, package-channels procedure will return at least
two channels. Take it into account.

* gnu/ci.scm (cuirass-jobs): Fix channels subset argument.

gnu/ci.scm

@@ -527,11 +527,10 @@ (define source
           (let ((all (all-packages)))
             (filter-map
              (lambda (package)
-               (match (package-channels package)
-                 ((channel . _)
+               (any (lambda (channel)
                       (and (member (channel-name channel) channels)
                            (package->job store package system)))
-                 (else #f)))
+                    (package-channels package)))
              all)))
          (('packages . rest)
           ;; Build selected list of packages only.