Fix clean-webpack-plugin

[slint]

Package version (if known): 1.2.5

Describe the bug

There's an error/warning in the build logs coming from the clean-webpack-plugin, potentially affecting cleaning up the dist folder after building assets.

Steps to Reproduce

1. Run invenio assets build
2. See the following error in the logs:

...
clean-webpack-plugin: dangerouslyAllowCleanPatternsOutsideProject requires dry: false to be explicitly set. Enabling dry mode
...

Expected behavior

Probably since no one complained yet, it's not that critical, but seems like a small change in the webpack.config.js to add dry: true.