Export QTILE_XEPHYR so configs can tell when they're run under Xephyr
Export QTILE_XEPHYR so configs can tell when they're run under Xephyr

For example, I plan to do:

if os.environ.get("QTILE_XEPHYR"):
    width = 480
    mod = "mod1"

So I can more easily test under Xephyr using only part of the screen,
and using a different mod key.

ramnes/qtile,qtile/qtile,qtile/qtile,ramnes/qtile