matplotlib Invalid DISPLAY variable

Broken nightlies in https://gitlab.idiap.ch/bob/bob.nightlies/-/jobs/87312

bob.bio.base.test.test_scripts.test_evaluate_closedset ... /home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/matplotlib/__init__.py:1405: UserWarning: 
This call to matplotlib.use() has no effect because the backend has already
been chosen; matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

  warnings.warn(_use_error_msg)
ERROR

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ERROR: bob.bio.base.test.test_scripts.test_evaluate_closedset
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Traceback (most recent call last):
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/bob/bio/base/test/test_scripts.py", line 490, in test_evaluate_closedset
    main(parameters)
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/bob/bio/base/script/evaluate.py", line 379, in main
    raise RuntimeError("During plotting of ROC curves, the following exception occured:\n%s" % e)
RuntimeError: During plotting of ROC curves, the following exception occured:
Invalid DISPLAY variable

======================================================================
ERROR: bob.bio.base.test.test_scripts.test_evaluate_openset
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Traceback (most recent call last):
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/bob/bio/base/test/test_scripts.py", line 516, in test_evaluate_openset
    main(parameters)
  File "/home/gitlab-runner/builds/0d638152/0/bob/bob.nightlies/build-prefix/lib/python2.7/site-packages/bob/bio/base/script/evaluate.py", line 460, in main
    raise RuntimeError("During plotting of DIR curves, the following exception occured:\n%s")
RuntimeError: During plotting of DIR curves, the following exception occured:
%s