cache reset index doesn't apply to CACHE_FALLBACK_KEY

For reproduce:

  1. Set CACHE_FALLBACK_KEY to master
  2. Click "Clear runner caches" in pipelines page
  3. Run some job.

Runner will try download cache: default-1, then master (without index).

Expected behaviour one of:

  1. Auto add reset index to falback key too (prefer).
  2. Add predefined variable: CI_CACHE_RESET_INDEX - for available manual add index to fallback key.
Edited by 🤖 GitLab Bot 🤖