This seems to build fine for me.
We don't need Python to build under Nix, is that correct?
Do we skip the part where Python would be used?
Note that I had to add a patchPhase before the buildPhase to replace /bin/bash and uname by their binaries in the nix-store.
I'm not sure if you want to add PHOENIX_BASH and PHOENIX_UNAME. If you do, we can remove the patchPhase again. I don't wanted to burden the build scripts any further with nix specifics, so I thought patching it in the nix files is the way to go.