make all OBS pdflatex builds reproducible

[bmwiedemann]

without the FORCE_SOURCE_DATE variable, packages like libvmime varied
because pdflatex ignored the SOURCE_DATE_EPOCH value

[bmwiedemann]

ping

[bugfinder]

is there a reason why tex uses a different variable ?

[bmwiedemann]

http://tug.org/pipermail/pdftex-commits/2016-June/000003.html and
https://reproducible-builds.org/blog/posts/59/ has details:

Some upstream build tools (e.g. TeX, see below) have expressed a desire to control
which cases of embedded timestamps should obey SOURCE_DATE_EPOCH.
They were not convinced by our arguments on why this is a bad idea, 
so we agreed on an environment variable FORCE_SOURCE_DATE
for them to implement their desired behaviour - named generically,
so that at least we can set it centrally.