Use BTreeMap instead of HashMap for source bundle manifest

[Jake-Shadle]

I noticed this when reimplementing some of sentry-cli's upload-difs portions, basically the exact same input objects (in our case wasm binaries + source bundles) would always say there were missing chunks on the server, The cause of which was the use of a HashMap to store the files in the source bundle manifest, which will give non-deterministic output with the same input due to the default HashMap hash randomization. Changing to use a BTreeMap gives deterministic ordering so that the same input object will always give the same output source bundle.

Also just want to note, the only reason that this is the case is that PR #470 changed the zip crate to remove default features, one of which is time, which when not present will always use 1980-01-01 for the timestamp.