oe/copy_buildsystem: check_sstate_task_list also pop BBPATH from env

oe/copy_buildsystem: check_sstate_task_list also pop BBPATH from env

The BBPATH environment could be set and can make a failure when try
to build an extensible sdk because it will look the bitbake.lock
file in the original build folder.

Example:

$ export BBPATH=`pwd`
$ bitbake core-image-minimal -c populate_sdk_ext

ERROR: bitbake failed:
ERROR: Only one copy of bitbake should be run against a build directory
ERROR: core-image-minimal-1.0-r0 do_populate_sdk_ext: Function failed:
copy_buildsystem

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>