Revert "guix system: Remove unused 'read-operating-system' procedure."

This reverts commit d11a432adf.

The 'read-operating-system' is used in Emacs-Guix.

gnu/ci.scm

@@ -38,6 +38,7 @@
   #:use-module ((guix licenses)
                 #:select (gpl3+ license? license-name))
   #:use-module ((guix utils) #:select (%current-system))
+  #:use-module ((guix scripts system) #:select (read-operating-system))
   #:use-module ((guix scripts pack)
                 #:select (self-contained-tarball))
   #:use-module (gnu bootloader)

guix/scripts/system.scm

@@ -92,6 +92,7 @@
   #:use-module (ice-9 match)
   #:use-module (rnrs bytevectors)
   #:export (guix-system
+            read-operating-system
 
             service-node-type
             shepherd-service-node-type))
@@ -107,6 +108,11 @@
                       (gnu services)
                       (gnu system shadow))))
 
+;; Note: The procedure below is used in external projects such as Emacs-Guix.
+(define (read-operating-system file)
+  "Read the operating-system declaration from FILE and return it."
+  (load* file %user-module))
+
 
 ;;;
 ;;; Installation.