Esteemed maintainers,

Currently, Calibre can't open .epub files unless `qtwebengine` package is also available in one's $GUIX_PROFILE. Neither can the stand-alone `ebook-viewer` program supplied with Calibre. It exits with the complaint: "Could not find QtWebEngineProcess".

Attached is a patch to fix the issue. QtWebEngineProcess is now made available to all Calibre binaries via QTWEBENGINE_PATH set during a new 'wrap-program phase introduced with the patch.

Thank you