longer-term, i want hosts/by-name to define host-specific data
that's accessible via the other hosts (things like pubkeys).
also the secrets management needs some rethinking. there's really not
much point in me specifiying where *exactly* a secret comes from at its
use site. i should really be specifying secret store manifests; i.e.
"servo.yaml contains secrets X Y and Z", and leaving the rest up to
auto-computing.
- shorter.
- congruent with `nixos-rebuild .` choosing what to build based on `hostname`.
- more widely used within other nix repos i've seen.
- more accurate in the case that i migrate a host to a different
machine (which i plan to do with servo).
