externalsrc: Fix a source date epoch race in reproducible builds

When reproducible builds are enabled and externalsrc is in use, the
source date epoch function is added. The conditions on the conditional
code removing the unpack task need to match the deltask function, else
the source date epoch function can end up running twice and the functions
can race with each other causing build failures or corruption.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e82095c0)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>