tools: zip: make encrypted archives reproducible

Zip always try to generate new encryption header depending on execution
time and process id, which is far from being reproducible. This commit
changes the zip srand() seed to a predictable value to generate
reproducible random bytes for the encryption header. This will compromise
the goal of secure archive encryption, but it would not be a big problem
for our purpose.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>