terminal: Exclude BB_ORIGENV from the signatures

devshell was printing a traceback when exiting due to the use of dump_sigs()
being called on the task. This is turn was since this function referenced
BB_ORIGENV. We might as well globally exclude this for now since its a
data store object and cannot be pickled, not would it make sense to do so.

[YOCTO #5683]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>