[Notifications] Read BAMF_DESKTOP_FILE_HINT from environment

This is a similar situation we have with Flatpak when the desktop entry the application thinks it owns is actually different.
In Snap this environment variable is passed to the process. When no service was found, try reading it.

CHANGELOG: Improved notification identification for Snap applications

Differential Revision: https://phabricator.kde.org/D21881