[Cython] new Jenkins setup

[Stefan Behnel]

Hi,

I moved the Jenkins installation out of the USB disk and into my home
directory. The USB disk has proven very fragile in the past, so this will
make us more independent from reboots and disk failures.

To keep the builds fast, the workspaces have moved into a ramdisk, which is
limited to 20 GB. This is less than the previous directory size, so I
changed the job configs to delete redundant data after the builds, namely
the unpacked CPython directories and the Cython installation. Those are
still available in the job workspaces in form of the archives that the
build jobs copy over at the beginning. So, to reproduce test failures on
the Jenkins server, you can just unpack them manually.

Currently, we are way below the limit (<5GB), but the developer branches
haven't been built yet. It looks like no job takes more than 1GB when it
runs, so the 6 active jobs that we run in parallel will not take more than
6GB in total. That leaves some 14 GB for the resident jobs. Those tend to
stay around 30-100MB each (the Cython matrix jobs are more like 30-50MB per
configuration), so we can keep quite a lot of jobs in the ramdisk. I'll
keep an eye on them from time to time, but I think we'll be fine with that
for a while.

Still, please take a bit of care when you make changes to build jobs that
you do not leave unnecessarily large sets of redundant data lying around at
the end. Anything that we clearly no longer need after the build and that
can be deleted will keep space free for running jobs and things like ccache.

Stefan