build - respect `SOURCE_DATE_EPOCH`

[cscheid]

This is something we should consider for our builds: https://reproducible-builds.org/specs/source-date-epoch/

[winniehell]

side note: matplotlib already supports this for some output formats (https://matplotlib.org/stable/users/prev_whats_new/whats_new_2.1.0.html#reproducible-ps-pdf-and-svg-output)

(this is helpful when caching figures)