Add global variable to access the challenge within Jinja scripts

[kkharbas]

Would be helpful to add a global variable carrying the challenge to use in Jinja scripts.

e.g.

"indices": [
    {
      "name": "msmarco-v2",
      {%- if challenge == 'hybrid-search-queries-dsl-and-esql'-%}
      "body": "index-vectors-with-text-mapping.json"
      {%- else -%}
      "body": "index-{{ mapping_type | default("vectors-only") }}-mapping.json"
      {%- endif -%}
    }
  ]