icecc-create-env: Add /bin/true to environment

icecream daemons execute /bin/true from the environment as a check to
determine if the environment is valid at all, so it needs to be
included.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>