weather: Fix type error when reporting CI stats.

* guix/scripts/weather.scm (report-server-coverage): Remove 'missing'
binding above 'queued-subset' call.  Before that 'queued-subset' would
be called with a number instead of a list.

guix/scripts/weather.scm

@@ -208,10 +208,10 @@ unavailable)~%"))
                                   0 queue)))
           (newline)
           (unless (null? missing)
-            (let ((missing (length missing)))
             (match (queued-subset queue missing)
               (#f #f)
               ((= length queued)
+               (let ((missing (length missing)))
                  (format #t (G_ "  ~,1f% (~h out of ~h) of the missing items \
 are queued~%")
                          (* 100. (/ queued missing))