There is interest in supporting Mirage OS and Solo5 unikernels but neither ambitions for Nixpkgs have matured. If you would like to see unikernel Linux support you are welcome to work on it. Unfortunately when working on exotic platforms you will be essentially working alone, but I would encourage you to go for it if you find it interesting.
Ultimately though, the only provider of the Linux API/ABI with traction is plain old Linux, because as Martin Děcký says, “if you want Linux you know where to find it”.