reproducible_build: Work around caching issues

SOURCE_DATE_EPOCH can be expanded early in the parsing process before
the class extensions are applied. This can mean the directory pointed
to for the SDE can be incorrect until later in parsing. Cache the file
name in the cached value and allow it to dynamically update.

This isn't ideal but avoding expansion of the variable likely isn't
possible and I'm not sure how else to handle this. This works around
the issue until a better solution can be found.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 44dc97cd)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>