perf: null out zip reader after reading archive

What does this MR do?

a.archive is only used inside readArchive which means we can null out the entire zip.Reader to recycle memory

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines