If using Bifrost to build images via diskimage-builder, the
dib_os_release variable can be used to specify the DIB_RELEASE
environment variable. However, if the dib_os_release variable is unset,
image creation will fail.
This change sets the dib_env_vars_final variable unconditionally to
ensure that it is always defined when accessed.
Change-Id: If32b2ebc13fe521e986ed6593fd3a8b99ec90567
Closes-Bug: #1665671