Only load the `pdfjs-overrides.css` bundle when the client is loaded in
PDF.js.

This fixes an issue where a global styling change that was recently
added specifically for PDF.js [1] ended up affecting other web pages
[2].

[1] https://github.com/hypothesis/client/pull/4208
[2] https://github.com/hypothesis/product-backlog/issues/1270#issuecomment-1039647361