`get_os_name` does not recognise Gentoo Prefix

[ocaisa]

Not sure how to solve this one, but for the case of EESSI ideally when reporting eb --show-system you would show that the Gentoo Prefix is being used.

This is perhaps debatable (at the end of the day there is an underlying OS), but it should be noted that EasyBuild can have different behaviour based on the information provided there,

[boegel]

How about just making EasyBuild aware of being run in the EESSI build environment, at least for --show-system-info?

It could report both the host OS, but also info on the EESSI compat layer.

I'm not sure making EasyBuild aware of being used in a generi Gentoo Prefix environment is worth it, but perhaps that can be done in one ago along with picking up on EESSI-specific things.