Set SOURCE_DATE_EPOCH when buliding outputs

[alerque]

Given the relatively large assortment of builds involved in a project I think the build dates ending up in PDFs and even images is getting to be relatively meaningless. The timestamps in the file and even on the output is a distracting difference that makes it harder to know if there are any real differences.

I think we should be setting SOURCE_DATE_EPOCH universally during our builds. This probably means extending the shell wrapper used for make targets to first sample all the input files being passed to the target, finding the newest one, then exporting the env var before running the rest of the commands from the target.

The biggest question will be whether to include project-local dependencies (e.g. as found and processed with git warp-time) or all the inputs (including system Lua libraries, fonts, etc.)

With that done we should review anywhere we use the system date ourselves and make sure that it checks for the environment variable first.

c.f. https://fosstodon.org/@pandoc/112529242207941776