toaster: fix path to buildstats file

toaster: fix path to buildstats file

The buildstats file path changes based on the
optional PE variable that may be defined for a
recipe.

The toasterui simply ignored the PE value, and
as such it didn't correctly reach buildstats files
for some of the tasks.

This patch fixes the issue.

    [YOCTO #5073]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>