ci: Upload artifacts on failure without specifying a directoryHEADmaster

`on_failure` allows to retrieve artifacts in case of failure, and only
in that case: see
https://docs.gitlab.com/ee/ci/yaml/index.html#artifactswhen

The build takes place in a directory of the form
`xfconf-${version}git${commit}/_build/sub` at the moment, but the
`_build/sub` part could possibly change in the future, as it is added by
Gitlab. So the easiest and most robust way seems to be to recover the
`test-suite.log` files without worrying about the path.

See also https://gitlab.xfce.org/xfce/xfconf/-/merge_requests/19#note_43611

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