dune: format root dune file (blank line after comment)

dune's @fmt formats dune files too; the warning-58 env stanza I added needed
a blank line after the comment block. Restores lint-fmt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: David Scott <dave@recoil.org>

Author: djs55

dune

@@ -2,6 +2,7 @@
 ; consuming virtual libraries (mirage-mtime, mirage-sleep, ...) which ship a
 ; .cmi but no .cmx, and ocaml-ci builds with -warn-error so it becomes fatal.
 ; It is a benign cross-module-inlining hint, not a correctness issue.
+
 (env
  (_
   (flags