offload: Disconnect sessions created by 'machine-load'.

This fixes a memory leak that can be seen by running:

  (map (lambda _ (machine-load m)) (iota 1000))

* guix/scripts/offload.scm (machine-load): Add call to 'disconnect!'.

guix/scripts/offload.scm

@@ -400,6 +400,7 @@ allowed on MACHINE.  Return +∞ if MACHINE is unreachable."
                                      "cat" "/proc/loadavg"))
             (line (read-line pipe)))
        (close-port pipe)
+       (disconnect! session)
 
        (if (eof-object? line)
            +inf.0 ;MACHINE does not respond, so assume it is infinitely loaded