Change fixed-size-thread-pool to a thread-pool

To avoid the queueing fiber.

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

@@ -2921,18 +2921,14 @@ WHERE builder != 'builtin:download'
                                  ignore-systems ignore-targets
                                  inferior-memory-limit)
   (define utility-thread-pool
-    (make-fixed-size-thread-pool parallelism))
+    ;; Use a thread pool rather than a fixed size thread pool, since this
+    ;; takes care of queuing waiters
+    (make-thread-pool parallelism))
   (define call-with-utility-thread
-    (let ((queued-channel
-           ;; There might be high demand for this, so order the requests
-           ;; TODO This doesn't shut down
-           (spawn-queueing-fiber
-            (fixed-size-thread-pool-channel utility-thread-pool))))
-      (lambda (thunk)
-        (call-with-thread
-         utility-thread-pool
-         thunk
-         #:channel queued-channel))))
+    (lambda (thunk)
+      (call-with-thread
+       utility-thread-pool
+       thunk)))
 
   (define (read-derivations filenames)
     (call-with-utility-thread