offload: Exit with code 100 upon build failures.

* guix/scripts/offload.scm (transfer-and-offload): Exit with code 100
  upon build failure.

guix/scripts/offload.scm

@@ -350,7 +350,10 @@ (define* (transfer-and-offload drv machine
                     (derivation-file-name drv)
                     (build-machine-name machine)
                     (status:exit-val status))
-            (primitive-exit (status:exit-val status)))))))
+
+            ;; Use exit code 100 for a permanent build failure.  The daemon
+            ;; interprets other non-zero codes as transient build failures.
+            (primitive-exit 100))))))
 
 (define (send-files files machine)
   "Send the subset of FILES that's missing to MACHINE's store.  Return #t on