gnu: dmd: Avoid "already exists" error on startup.

* gnu/system/dmd.scm (dmd-configuration-file)[config]: Remove
  "/var/nix/gcroots/etc-directory" before trying to recreate it.

gnu/system/dmd.scm

@@ -305,6 +305,7 @@ (define config
                               (not (member file '("." ".."))))))
 
          ;; Prevent ETC from being GC'd.
+         (rm-f "/var/nix/gcroots/etc-directory")
          (symlink ,etc "/var/nix/gcroots/etc-directory"))
 
        (format #t "starting services...~%")