Make `overmind` the default process

[amitaibu]

So the project's tests could run on GH actions, without needing to define it in the project's flake.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 😄

[mpscholten]

👍

The IHP devenv shell is configured here https://github.com/digitallyinduced/ihp/blob/master/flake-module.nix#L194