Fix non-deterministic output for merge_zips

A map iteration was causing a non-deterministic order
in output entries. The default behaviour is to preserve
the same order of the input zips, which is necessary
to ensure a deterministic output for deterministic inputs.

Bug: b/322788229
Test: Ran a couple of builds and confirmed no cache misses from
the output of merge_zips.

Change-Id: I3217e6887ab108d213a290b59da5b33d51b8241f