Add basic NixOS support

As

maintainer

I would like to

make the test suite of Rex pass in NixOS

so I can

enable users to use basic Rex features on these systems

Additional context

We have previously experimented with this together with @qbit, and the results are in the nixos branch.

Describe the solution you would like

I would like to focus on making sure that at least the test suite is passing on NixOS, and Rex can recognize NixOS systems.

Preliminary results suggest it's not that far as we initially thought, and some of the slight changes required for this may also add general benefits (e.g. not assuming path for common CLI tools).

Further support, like package and service management may be added separately (even outside Rex core as a separate CPAN distribution).

Describe alternatives you have considered

One alternative is to add only generic changes to Rex core, without the bits specific to NixOS. That would still make Rex code more robust when having to handle more unexpected/esoteric system.

Start by reviewing the existing nixos branch and running Rex's test suite on NixOS. Identify the changes needed for the suite to pass and for Rex to recognize NixOS systems, while keeping package and service management out of scope. Done means the test suite passes on NixOS and system recognition works.