feat: add buildbot_zisi_trace_nft feature flag

[eduardoboucas]

Summary

Adds a buildbot_zisi_trace_nft feature flag and passes it to zip-it-and-ship-it as traceWithNft (see netlify/zip-it-and-ship-it#769).

It also concentrates all feature flag assertions via sinon into one test. These tests have to run with .serial, so the fewer we have, the better.

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[ehmicky]

Great!