Found the commit where we had configured the work dir in the past: https://github.com/xwiki/xwiki-platform/commit/fde88e63e7884a9daf7c069b672088a99ca01742

It was removed in https://github.com/xwiki/xwiki-platform/commit/203e1e8c67ad74a420c91754c474cc0d640219d4 which is for issue XWIKI-7888 where I said:

And remove setting the work dir so that Jetty uses the system temporary dir, see http://docs.codehaus.org/display/JETTY/Temporary+Directories

So this was done on purpose, to rely on the system tmp dir, in order to make it more standard probably.

hmm we now need to decide what we want to do....