Save pending changelists as buildkite artefacts

[ca-johnson]

Not 100% sure about this, but it might be better than the current method of:

1. Make a copy of shelved cl
2. Save cl number in BK metadata
3. Prefer this over the users original, for stricter versioning
   Possible benfits:
   A) Less load on p4 server, no changelists that hang about forever
   B) Faster vs p4 print -o for each file (maybe?)

Major Caveat: Loses ability to iterate quickly on a given step by retrying it without the need to run the entire pipeline again