Compress precompiled python zips

When making this script deterministic we accidentally changed it so that
none of the entries in the output zip file were compressed. Re-add
the compression.

Adding the compression makes atest go from 67M to 34M, at the cost of
0.33s startup time instead of 0.28s.

Test: Presubmits
Change-Id: I4e58eb76235ddabf0195694cdd3644f26b7e84ec