ci: Add JOB_ARTIFACTS_BASE variable

Used for both LAVA (uploading results to MinIO because we don't yet have
non-ephemeral NFS storage) and Piglit (for the Tracie dashboard).

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11309>

.gitlab-ci.yml

@@ -8,6 +8,8 @@ variables:
   MINIO_HOST: minio-packet.freedesktop.org
   # per-pipeline artifact storage on MinIO
   PIPELINE_ARTIFACTS_BASE: ${MINIO_HOST}/artifacts/${CI_PROJECT_PATH}/${CI_PIPELINE_ID}
+  # per-job artifact storage on MinIO
+  JOB_ARTIFACTS_BASE: ${PIPELINE_ARTIFACTS_BASE}/${CI_JOB_ID}
 
 include:
   - project: 'freedesktop/ci-templates'