Use stdbuf instead of unbuffer for CI timestamping

Varacrequested to merge
(removed):ci_fail_when_tests_break into master

There are different reasons for this:

  • Using stdbuf will correctly return and non-zero exit code so when something breaks during CI gitlab-runner will mark the build as failed (Resolves: #8821 (closed)).
  • stdbuf is already installed by the coreutils package and thus saves diskspace

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