Fix picle error on ota_from_target_files

When trying to make a deep copy of PartitionBuildProps, execution fails
with picle error because PartitionBuildProps stores a ZipFile object,
which stores a lock object, which doesn't support pickling.

To fix it, pickle ZipFile object as a str to its path.

Test: generate ota with boot variable files
Bug: 253549364
Change-Id: I92c5d78ce65045495203c85862922d33d886e9ea