Fix PWD issue with new exec_func_shell

Fix PWD issue with new exec_func_shell

The previous attempt was incorrect.  The issue isn't that subprocess fails to
set PWD, it's that PWD is in the metadata, inherited from the environment, and
is re-exported, overwriting the actual accurate one in the shell environment
with the old one from the metadata.  So, ensure that PWD in the metadata is
not exported.

We can ditch this when the environment handling is reworked (e.g. poky's
commit to do so).

Signed-off-by: Chris Larson <chris_larson@mentor.com>