lint: Use the 'warning' procedure for messages.

* guix/scripts/lint.scm (emit-warnings): Use 'warning' instead of
'format'.

guix/scripts/lint.scm

@@ -46,10 +46,9 @@ (define (emit-warnings warnings)
    (lambda (lint-warning)
      (let ((package (lint-warning-package lint-warning))
            (loc     (lint-warning-location lint-warning)))
-       (format (guix-warning-port) "~a: ~a@~a: ~a~%"
-               (location->string loc)
-               (package-name package) (package-version package)
-               (lint-warning-message lint-warning))))
+       (warning loc (G_ "~a@~a: ~a~%")
+                (package-name package) (package-version package)
+                (lint-warning-message lint-warning))))
    warnings))
 
 (define (run-checkers package checkers)