Support `dune ocaml` subcommands over RPC

## Expected Behavior

All `dune ocaml` subcommands work when an RPC server is running

## Actual Behavior

```
$ dune ocaml doc
Error: A running dune (pid: 3749451) instance has locked the build directory.
If this is not the case, please delete "_build/.lock".
↳[1]
```