This way we can actually build and run a node using just:
```sh
nix run 'github:status-im/nimbus-eth2?submodules=1'
```
The `?submodules=1` part should eventually not be necessary.
For more details see:
https://github.com/NixOS/nix/issues/4423
Signed-off-by: Jakub Sokołowski <jakub@status.im>
Other changes:
* Include java in the Nix development shell
(so users are able to run the web3signer)
* Update the simulation config files to the latest spec
(so the web3signer accepts them)