Just like `--chain=gdev_live` allows to redefine the client specs using `CLIENT_SPEC` environment variable, we should be able to do the same using `--chain=<file>`. This is mandatory to be able to release a new client with just the client specs being changed (while the genesis remains identical). See [this comment](https://git.duniter.org/nodes/rust/duniter-v2s/-/merge_requests/297#note_46254) for more details.