Set QT_PLUGIN_PATH, QML_IMPORT_PATH, and QML2_IMPORT_PATH during build
so that Qt programs may easily be wrapped for use outside NixOS. The
paths are set appropriately for the dependencies of the package.
This disables propagated of Qt 5 build inputs at the request of
@edolstra. If this breaks your build, you need to add the missing
dependencies yourself. If this breaks your package at runtime, you need
to wrap your package to set either the plugin path or the QML import
path.
