Fix CI tests

[chrisburr]

The CI started failing as elastic search stops accepting more data once disk usage is at 95%. For some reason the amount of available disk space has been decreasing recently causing this to now happen (actions/runner-images#709).

I've also added a stage for showing the logs from elasticsearch to make problems easier to debug in future.

[matthewfeickert]

@chrisburr Just wanted to mention that the pyhf dev team has found it is enough to run apt-get clean before installing things to have a hack fix.

[chrisburr]

Hey @matthewfeickert! Thanks that will probably be useful in the other repo where I had the same issue and where I actually need the extra space.

I think this PR is still the right fix for the tests here as the CI doesn't need to care what percentage of disk space is remaining. The tests don't produce that much data in elasticsearch and we already reduce the memory reservation for the same reason.

[fstagni]

@chrisburr nothing prevents to add in here also apt-get clean as suggested (https://github.com/scikit-hep/pyhf/pull/819/files#diff-e9f950f17198d3d5e3122a44230a09b9R29)

[chrisburr]

I thought it would be better not to as it's a hack around a temporary issue with the runners. By the same logic we should remove other large objects that we don't use (boost/65 versions of the .NET SDK/44 versions of the Android SDK/...). It's worth it if we need the extra space but not if we don't.

The changes in this PR will always make sense as GitHub only promises that 14GB of disk will be available. As the runners provide a reasonably complete environment it's plausible that ~100GB will already be used making it very easy to trigger the 95% threshold despite having plenty of disk space available.

[fstagni]

apt-get clean is innocuous but OK let's leave it out.