gnu: guix-build-coordinator: Support cross-compilation.

Due to fibers not supporting the Hurd yet, the coordinator itself won't work,
but the agent should.

* gnu/packages/package-management.scm (guix-build-coordinator)[inputs]: Don't
include sqitch if building for the hurd.
[propagated-inputs]: Don't include guile-fibers if building for the hurd.

gnu/packages/package-management.scm

@@ -1097,9 +1097,13 @@ (define-public guix-build-coordinator
       (inputs
        `(("guile" ,@(assoc-ref (package-native-inputs guix) "guile"))
          ("sqlite" ,sqlite)
-         ("sqitch" ,sqitch)))
+         ,@(if (hurd-target?)
+               '()
+               `(("sqitch" ,sqitch)))))
       (propagated-inputs
-       `(("guile-fibers" ,guile-fibers)
+       `(,@(if (hurd-target?)
+               '()
+               `(("guile-fibers" ,guile-fibers)))
          ("guile-prometheus" ,guile-prometheus)
          ("guile-gcrypt" ,guile-gcrypt)
          ("guile-json" ,guile-json-4)