It's seems that the maintainer of https://www.ijg.org changed the source
files of libjpeg (even without changing the timestamp) [1]. We could
either update the md5sum or change the source link. Since this may
happen again it seems to be less maintenance efforts to change the
source to a more reliable, less frequently changing source (unless we
explicitly want the update). Because we also use buildout.net in other
components [2], this looks like an acceptable replacement.

[1] https://github.com/conan-io/conan-center-index/issues/4151
[2] https://lab.nexedi.com/nexedi/slapos/blob/a86050ed/component/libcap-ng/buildout.cfg#L9