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/pkg-snapshot

@@ -4,7 +4,7 @@
 # The diff doubles as a log of what you added and when.
 set -euo pipefail
 REPO="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
-HOST="${DOTFILES_HOST:-$(hostname | tr '[:upper:]' '[:lower:]')}"
+HOST="${DOTFILES_HOST:-$(uname -n | tr '[:upper:]' '[:lower:]')}"
 OUT="${REPO}/packages"
 mkdir -p "$OUT"