You don't have to specify the nixosConfiguration name (e.g ~/flake#my-configuration)? Do you just name it "default" or something for that to work? I've never tried that.
You name it the same as your current system's hostname.
Oh, I had no idea. I was actually already doing that. Guess I was just being extra verbose on the command line.
Exactly as Macha said, if you name the attribute `nixosConfiguration.<hostname>`, `nixos-rebuild` will look for that attribute implicitly; you can still specify another configuration if you're e.g. in a VM or a live image.