Set environtment for flatpak run

Problem: journalctl -xe: Exception: Anki requires a UTF-8 locale.

Soltion: change

/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=anki --file-forwarding net.ankiweb.Anki @@ %f @@

to

/usr/bin/flatpak run --branch=stable --env=LANG=en_US.UTF-8 --arch=x86_64 --command=anki --file-forwarding net.ankiweb.Anki @@ %f @@