toaster: unlock BuildEnvirnoment when build is done

toaster: unlock BuildEnvirnoment when build is done

There is no need to lock build environment before changing
build status as this operation is very fast. However, there
is a need to unlock it after changing build status.

Explicitly unlocked BuildEnvironment after build reaches
final status SUCCEEDED, FAILED or CANCELLED. This should
allow runbuilds process to pickup next build faster.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>