Add Task Id to the environment variables

I have looked around and even peeked into the source code(not good enough to
change it myself yet) but I can't seem to find any indication that the Task Id
is placed in the env. This would be a very handy thing to have to identify
specific task runs in the build itself

I hope this isn't a duplicate, I checked the issue tracker and mailing lists but
if it is sorry.

-martin