offload: Wait for the processes involved in 'guix archive --missing'.

* guix/scripts/offload.scm (send-files): Keep the second return value of
  'filtered-port'.  Call 'waitpid' on it.

guix/scripts/offload.scm

@@ -26,6 +26,7 @@
   #:use-module ((guix build utils) #:select (which mkdir-p))
   #:use-module (guix ui)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
@@ -356,8 +357,10 @@ with exit code ~a~%"
 success, #f otherwise."
   (define (missing-files files)
     ;; Return the subset of FILES not already on MACHINE.
-    (let* ((files   (format #f "~{~a~%~}" files))
-           (missing (filtered-port
+    (let*-values (((files)
+                   (format #f "~{~a~%~}" files))
+                  ((missing pids)
+                   (filtered-port
                     (list (which %lshg-command)
                           "-l" (build-machine-user machine)
                           "-p" (number->string (build-machine-port machine))
@@ -365,6 +368,7 @@ success, #f otherwise."
                           (build-machine-name machine)
                           "guix" "archive" "--missing")
                     (open-input-string files))))
+      (for-each waitpid pids)
       (string-tokenize (get-string-all missing))))
 
   (with-store store