Use uname -n instead of hostname(1)

The NAS is a minimal Arch install with no inetutils, so hostname(1) does
not exist and install died on its first line of real work. uname -n is
POSIX and always present. The same assumption was in dots, pkg-snapshot,
pkg-restore and zshenv's fallback.

common/zsh/zshenv

@@ -2,7 +2,7 @@
 # Keep it cheap: no subshells in the hot path, no vault lookups, no network.
 
 export DOTFILES="${DOTFILES:-$HOME/.dotfiles}"
-: ${DOTFILES_HOST:=${(L)${HOST:-$(hostname)}}}   # lowercased; SCTFW004 -> sctfw004
+: ${DOTFILES_HOST:=${(L)${HOST:-$(uname -n)}}}   # lowercased; SCTFW004 -> sctfw004
 export DOTFILES_HOST
 
 export EDITOR="nvim"