reproducible dpk archive build

[illwieckz]

Issue #2 tracked the effort to allow anyone to rebuild assets in the way they get something with the very same quality and level of completeness… but produced file are not truly reproducible bit to bit.

This is a (growing) list of things to fix in order to produce bit-to-bit reproducibility:

• [x] q3map2 build comment in bsp file (it's a job for q3map2);
• [ ] file timestamp in zip archive (it's a job for Urcheon);
• [ ] recompression optimization in Urcheon (I currently use advzip -r -k -4 <dpk file> to get smaller file size than Python's zipfile produces);
• [ ] standardized way to include nexe in unvanquished dpk;
• [ ] nexe reproducibility for unvanquished dpk, what's the status on it? It's well known cmake is not good for reproducibility.

[necessarily-equal]

I believe the first one is addressed by https://gitlab.com/xonotic/netradiant/-/merge_requests/190