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: nodejs#37566
Fixes: nodejs#37560
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Ruy Adorno <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>

Author: lpinca