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/bin/dots

@@ -6,7 +6,7 @@
 # point: a login banner that always fires is a banner you stop reading.
 set -uo pipefail
 REPO="${DOTFILES:-$HOME/.dotfiles}"
-HOST="${DOTFILES_HOST:-$(hostname | tr '[:upper:]' '[:lower:]')}"
+HOST="${DOTFILES_HOST:-$(uname -n | tr '[:upper:]' '[:lower:]')}"
 NAG=0; [[ "${1:-}" == "--nag" ]] && NAG=1
 cd "$REPO" 2>/dev/null || exit 0