classes/populate_sdk_ext: ensure we clean the right temporary TMPDIR path

classes/populate_sdk_ext: ensure we clean the right temporary TMPDIR path

After we run the build system within the eSDK internally as part of the
sstate filtering that happens during do_populate_sdk_ext, we need to
ensure that the TMPDIR created during that process gets deleted. However
we were using the TMPDIR path for the build producing the eSDK which may
not be the same (since that value would typically be filtered out) thus
if the user had set TMPDIR to something other than the default, the
temporary TMPDIR would not be deleted which not only led to extraneous
junk entering the SDK but also failures during install because the
TMPDIR path was different. In order to fix this, force TMPDIR to a known
value during the sstate filtering run so we know what to delete
afterwards.

Fixes [YOCTO #10210].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>