ci: Remove any existing results directory before running piglit
A pre-existing results directory (from a previous job) could cause a spurious failure, see e.g. https://gitlab.freedesktop.org/mesa/mesa/-/jobs/4410206
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A pre-existing results directory (from a previous job) could cause a spurious failure, see e.g. https://gitlab.freedesktop.org/mesa/mesa/-/jobs/4410206