yubioath-desktop: prefix QML2_IMPORT_PATH

Otherwise, yubioath-desktop can't find pyothersides qml files:

QQmlApplicationEngine failed to load component
qrc:/qml/main.qml:168 Type YubiKey unavailable
qrc:/qml/YubiKey.qml:2 module "io.thp.pyotherside" is not installed

[1]    17017 segmentation fault  result/bin/yubioath-desktop