sources: Skip archiving step if the source tarball already exists
sources: Skip archiving step if the source tarball already exists

Partially reverts 53e07851fd32898dc9b5319fc2e5a15230ab88f0.

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@citrix.com>

djs55/planex,djs55/planex,djs55/planex