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

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6601>

.gitlab-ci/piglit/run.sh

@@ -5,6 +5,7 @@ set -o xtrace
 
 VERSION=`cat install/VERSION`
 
+rm -rf results
 cd /piglit
 
 PIGLIT_OPTIONS=$(echo $PIGLIT_OPTIONS | head -n 1)