Make `overmind` the default process

So the project's tests could run on GH actions, without needing to define it in the project's flake.nix

```nix
  devenv.shells.default = {
      # This is needed so when running tests in GitHub actions, we can execute `devenv up &` without an error.
      process.implementation = "overmind";
  };
```

Not sure where to set it on IHP, though :smile: 