Output doctest logs on nightly

[robertapplin]

Description of work
This PR should make it easier to debug issues with our nightly pipeline when it fails building the docs-doctest target. The output from this will now be put into a file in the build directory called doctest_$OS_TYPE.log, and then uploaded as an artifact at the end of the pipeline. This will mean it will appear in the "build artifacts" screen for the job. Previously, Xvfb would swallow any output from the building of docs-doctest and not show us anything. This has been avoided by using the QT_QPA_PLATFORM=offscreen variable instead of Xvfb.

To test:
Code review, and check that you can see a doctest_$OS_TYPE.log files in the build artefacts for this run. There will be no doctest_msys.log for windows because the doc tests do not get run on windows.
https://builds.mantidproject.org/job/build_packages_from_branch/509/

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[thomashampson]

This is a nice concise change to reduce the size of the build logs to something more manageable. However there is one problem I can see with the renaming of the ctest log files.

[thomashampson]

This looks to be working well. We just need to remember to archive the doctest logs on the PR builds on Jenkins