Testing workflow is not failing when the linter throws a warning

[josecelano]

Run npm run lint

> lint
> eslint --ext ".ts,.js,.vue" .


/home/runner/work/torrust-index-gui/torrust-index-gui/components/Markdown.vue
  3:8[5](https://github.com/torrust/torrust-index-gui/actions/runs/8800994255/job/24153350112?pr=535#step:4:6)  warning  Expected a space before '/>', but not found  vue/html-closing-bracket-spacing

✖ 1 problem (0 errors, 1 warning)
  0 errors and 1 warning potentially fixable with the `--fix` option.

The step is throwing a warning.

It's not failing because it's only a warning, but we should also make it fail with warnings.