tools: make update-eslint.sh work with npm@7

Install `eslint-plugin-markdown` at the same level of `eslint` without
cd'ing into `eslint` directory, otherwise the following error is raised:

```
npm ERR! code ERESOLVE
npm ERR! Cannot destructure property 'name' of 'node' as it is null.
```

PR-URL: https://github.com/nodejs/node/pull/37566
Fixes: https://github.com/nodejs/node/issues/37560


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruy Adorno <ruyadorno@github.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>