feat: add install script for nkeys

Installs the nk CLI from nats-io/nkeys releases. The release zip
nests everything under a nkeys-v<version>-<os>-<arch>/ directory
(binary is named nk, not nkeys), so set_link points at that nested
path rather than flattening the archive.

dot_local/bin/executable_install-nkeys

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+# shellcheck disable=SC1091
+source "${XDG_DATA_HOME}/buddy-up/includes/utils.sh"
+
+APP=nkeys
+VERSION=${VERSION:-0.4.16}
+DEST=${XDG_DATA_HOME}/apps/releases/${APP}
+
+main() (
+	ARCHIVE_DIR=nkeys-v${VERSION}-$(get_os)-$(get_arch)
+	URL=https://github.com/nats-io/nkeys/releases/download/v${VERSION}/${ARCHIVE_DIR}.zip
+
+	extract_zip "${URL}" "${DEST}/${VERSION}"
+	set_current_link "${DEST}" "${VERSION}"
+	set_link "${XDG_BIN_HOME}/nk" "${DEST}" "${ARCHIVE_DIR}/nk"
+)
+
+get_arch () (
+	case $(uname -m) in
+		x86_64*)    echo amd64 ;;
+		arm64*)     echo arm64 ;;
+		aarch64*)   echo arm64 ;;
+		*)          >&2 echo "unsupported uname: $(uname -m)"; exit 1 ;;
+	esac
+)
+
+get_os () (
+	case $(uname -s) in
+		Linux*)     echo linux ;;
+		Darwin*)    echo darwin ;;
+		*)          >&2 echo "unsupported os: $(uname -s)"; exit 1 ;;
+	esac
+)
+
+main "${@}"