[WIP] Build .pex files to speed up testing

[mrpau-eugene]

Summary

@benjaoming quite off from the instructions but I made it similar to how kolibri generates the pex and installers. I can make it as instructed (.buildkite/pex.sh) if this approach is not appropriate

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• [x] Generate pex file dist/ka-lite-static<version-info>.pex
• [ ] Activate script on Buildkite
• [ ] Get PR status feedback through bot

Should we also upload the files to google cloud storage?

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Issues addressed

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5544

[benjaoming]

@mrpau-eugene I found that the current Buildkite settings skip third-party PRs for security reasons.. which is actually a good idea.. however, just for now, let's switch it on so we can start using it and testing it some more.

Would you mind opening a new PR? Buildkite skips it despite that I close and re-open because it has seen the commit before methinks...