lint fix: rely on npm_ vars being set
lint fix: rely on npm_ vars being set

These vars should be assumed to be set by npm when this script is
invoked (since it's invoked by a hook script which should be invoked by
npm). However, since npm is using nonstandard lowercase, shellcheck
complains that the vars aren't explicitly set within the script.

Instead of implicitly assuming the vars are set, let's just blow up if
they aren't set. (which incidentally makes shellcheck happy)

nodenv/nodenv-package-rehash,jasonkarns/nodenv-package-rehash