Avoid choice-operation for the gc watcher

In case this causes a memory leak.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -23,6 +23,7 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 threads)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 atomic)
   #:use-module (ice-9 exceptions)
   #:use-module (ice-9 textual-ports)
   #:use-module (ice-9 hash-table)
@@ -3456,8 +3457,8 @@ SKIP LOCKED")
                                              ignore-targets
                                              inferior-memory-limit
                                              parallelism)
-  (define finished-channel
-    (make-channel))
+  (define finished?
+    (make-atomic-box #f))
 
   (define job-not-found-exception
     (make-exception-with-message
@@ -3504,13 +3505,8 @@ SKIP LOCKED")
                           (/ (assq-ref stats 'heap-free-size)
                              (expt 2. 20)))))
 
-              (when
-                  (perform-operation
-                   (choice-operation
-                    (wrap-operation (get-operation finished-channel)
-                                    (const #f))
-                    (wrap-operation (sleep-operation 3)
-                                    (const #t))))
+              (unless (atomic-box-ref finished?)
+                (sleep 3)
                 (loop gc-times
                       gc-time-taken))))))
 
@@ -3652,5 +3648,5 @@ WHERE revision_id = $1"
             "update-derivation-outputs-statistics"
           (update-derivation-outputs-statistics conn))))))
 
-  (put-message finished-channel #t)
+  (atomic-box-set! finished? #t)
   result)