Also accept strings as sendmail paths in backend Guix service

.guix/modules/datarekisteri-service.scm

@@ -70,6 +70,10 @@
            (service-extension account-service-type frontend-accounts)))
    (description "Run the datarekisteri HTTP frontend server")))
 
+(define (file-like-or-string? x)
+  (or (file-like? x)
+      (string? x)))
+
 (define-configuration/no-serialization plain-datarekisteri-backend-configuration
   (datarekisteri-backend
    (file-like datarekisteri-backend)
@@ -84,7 +88,7 @@
    (integer 3100)
    "The TCP port to listen on.")
   (sendmail
-   (file-like)
+   (file-like-or-string)
    "Path to the sendmail program to send emails with."))
 
 (define (backend-shepherd-service config)