The installer generates `nixosConfigurations.<hostname>` (see
installer/install.sh: `nixosConfigurations.$HOSTNAME`), but the system
update script was rebuilding `.#default` and using `--impure`. The
`#default` literal worked only on dev hosts that happened to be named
"default" and silently broke every toggle script on real installs.
Now resolves `$(hostname)` at runtime and aborts with a clear error if
empty. Dropped `--impure` — the flake doesn't need it.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
