Add User to b2qt service file

The b2qt.service is used for the auto start feature from Qt Creator.
Make sure it's using the same environment (especially HOME) as when
running the app from Qt Creator.

Fixes: QTBUG-79738
Change-Id: I4633914b0a80f2420942757261f2c8d9d6ecbcca
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>