nixos: provide /etc/hostname

/etc/hostname is the file used by hostnamectl(1) and the
org.freedesktop.hostname1 dbus service (both provided by systemd) to get
the "static hostname". Better provide it so that users of those
tools/services get a proper hostname.

An example of an issue created by the lack of /etc/hostname is that the
bluetooth stack on NixOS identifies itself to peers as "BlueZ $VERSION"
instead of the hostname.

References:
https://www.freedesktop.org/software/systemd/man/hostname.html

Changes v1 -> v2:
  * ensure /etc/hostname ends with a newline