Create a central nix file that defines nixpkgs version globally

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Add a central Nix file that defines the nixpkgs version and possibly the service packages globally. That way, we could avoid having to pin nixpkgs in each and every Nix file separately (which is a maintenance burden - what if we want to bump the version?) and we can be sure that the service packages (Redis, Kafka, ...) are the same in Nix shells and the tests. Another option would be to get as many dependencies as possible via an import from the Nix shells and add the ones that aren't specified in there (bash, coreutils, ...). *: for some services, there are multiple versions packaged in nixpkgs, e.g. for Kafka.