build now downloading whole zlib and libpng

[SelvinPL]

https://github.com/gbdev/rgbds-live/actions/runs/31914350792/job/95083946666#step:6:33

seems like "We set *_FOUND so find_package assumes that its job has already been (successfully) done," is not true
(at least without CACHE BOOL "" FORCE)

[SelvinPL]

or maybe set(ZLIB_LIBRARY "z" CACHE STRING "" FORCE) was preveting ... I don't remeber ... but that's why it was there ...