Darwin: inconsistencies between CI and Runtime

CI uses `nix flake check`, which calls the `process-compose` app wrapped inside `pkgs.runCommand`. The apps that run inside `runCommand` gets a few packages by default, for example `readlink` from `coreutils`. This behaviour could make the tests pass in CI but fail on runtime (when ran using `nix run`), if the service doesn’t pass those packages in its environment.

For more detailed analysis, see: https://github.com/juspay/services-flake/pull/164#issuecomment-2053876700