Staging Scheme code to run as one-shot service

["Owen T. Heisler" <writer@owenh.net>]

service where the entire service functionality is in a procedure. =====
Hi! I am trying to declare a simple one-shot operating-system service
where the entire service functionality is in a procedure. Following is
a minimal reproducible example. The error is `In procedure fport_write:
Broken pipe` (see end of message). What is wrong here? I'm new to
Scheme and Guix, and it's not very clear to me yet how code staging and
g-expressions work, so it's probably something quite obvious. Thanks
for looking!

```scm
;; operating-system declaration
(use-modules (gnu)
              (gnu services shepherd)
              (ice-9 popen))
(use-service-modules networking)
(use-package-modules bootloaders)
(define (say-hello)
   #~(let ((port (open-pipe* OPEN_WRITE "logger" "-plocal0.alert")))
       (display "========say-hello========\n" port)
       (close-pipe port)))
(define say-hello-service
   (simple-service 'say-hello-service shepherd-root-service-type
                   (list (shepherd-service (auto-start? #f)
                                           (documentation "Say hello.")
                                           (one-shot? #t)
                                           (provision '(say-hello-service))
                                           (respawn? #f)
                                           (start #~(lambda ()
                                                      #$(say-hello)))))))
(operating-system
   (bootloader (bootloader-configuration
                 (bootloader grub-bootloader)
                 (targets '("/dev/vda"))
                 (terminal-inputs '(console serial))
                 (terminal-outputs '(console serial))
                 (timeout 1)))
   (file-systems (cons (file-system
                         (mount-point "/")
                         (device "/dev/vda1")
                         (type "ext4")) %base-file-systems))
   (host-name "test-guix-vm")
   (kernel-arguments (cons "console=ttyS0" %default-kernel-arguments))
   (services
    (append (list say-hello-service) %base-services)))
```

```sh
# Build system image and run with qemu
cp $(guix system image --image-type=qcow2 in.scm) image.qcow2
chmod u+w image.qcow2
qemu-system-x86_64 -enable-kvm -m 2048 -device virtio-blk,drive=myhd \
     -nographic -drive if=none,file=image.qcow2,id=myhd
```

```console
## Log in as root and attempt to start service
# herd start say-hello-service
Starting service say-hello-service...
Service say-hello-service failed to start.
herd: error: exception caught while executing 'start' on service 'say-hello-service':
In procedure fport_write: Broken pipe
```