Retry computing channel instance manifests

On readlink errors.

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

@@ -1570,12 +1570,13 @@ SELECT store_path FROM derivation_source_files WHERE id = $1"
                           (commit ,(channel-commit channel)))
                  ,(channel-instance-commit channel-instance)
                  ,(channel-instance-checkout channel-instance)))))
+
+         (define (compute)
            (simple-format
             (current-error-port)
             "guix-data-service: computing the derivation-file-name for ~A\n"
             system)
 
-         (let ((manifest
            (catch #t
              (lambda ()
                ((channel-instances->manifest instances #:system system) store))
@@ -1587,7 +1588,16 @@ SELECT store_path FROM derivation_source_files WHERE id = $1"
                (simple-format
                 (current-error-port)
                 "error ~A: ~A\n" key args)
-                    #f))))
+
+               (match (cons key args)
+                 (('system-error "readlink" _ ...)
+                  'retry)
+                 (_ #f)))))
+
+         (let loop ((manifest (compute)))
+           (when (eq? mainfest 'retry)
+             (loop (compute)))
+
            (define (add-tmp-root-and-return-drv drv)
              (add-temp-root store drv)
              drv)