services: guix-build-coordinator: Output the start script name.

As this can be useful for debugging purposes.

* gnu/services/guix.scm (make-guix-build-coordinator-start-script): Output the
current filename.

gnu/services/guix.scm

@@ -200,6 +200,8 @@
                       (guix-build-coordinator build-allocator)
                       (guix-build-coordinator coordinator))
 
+         (simple-format #t "starting the guix-build-coordinator:\n  ~A\n"
+                        (current-filename))
          (let* ((metrics-registry (make-metrics-registry
                                    #:namespace
                                    "guixbuildcoordinator_"))