Add fmt constraint on ocluster.0.1 (spotted in ocaml#27602).

Author: dbuenzli

packages/ocluster/ocluster.0.1/opam

@@ -25,7 +25,7 @@ depends: [
   "capnp-rpc-net" {< "2.0"}
   "capnp-rpc-unix" {>= "0.9.0" & < "2.0"}
   "logs"
-  "fmt"
+  "fmt" {< "0.10.0"}
   "conf-libev" {os != "win32"}
   "digestif" {>= "0.8"}
   "fpath"