Set the root_dir in the OPAM state

If not set then OPAM does not set the right path for the OPAM root and
will use the default `~/.opam` no matter what. The OPAM API is
surprising in this regard that even passing the root folder to
`load_defaults` does not actually end up setting the folder.

Closes tarides#197

bin/opam_monorepo.ml

@@ -25,7 +25,7 @@ let init_opam () =
   OpamFormatConfig.init ();
   OpamCoreConfig.init ~safe_mode:true ();
   OpamRepositoryConfig.init ();
-  OpamStateConfig.init ()
+  OpamStateConfig.init ~root_dir:root ()
 
 let () =
   init_opam ();