tools: fix update-eslint.sh

The script currently assumes that there is a package.json in
`eslint-tmp`. If there isn't the logic of the script fails.
This adds a call to `npm init --yes` ensuring there is a package.json
and that the script can do it's thing.

PR-URL: https://github.com/nodejs/node/pull/14850


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>