[PWGHF] trackIndexSkimCreator: Avoid slicing inside the inner loop

[ktf]

The bachelor track indices depend only on the collision, not on the
cascade, so slice them once per collision. Each sliceBy on this Filtered
join does a full arrow Table::Slice (per-column allocation) plus a
selection-vector slice, so rebuilding it for every cascade is pure waste.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 8 warnings, 🔕 7 disabled

[vkucera]

Thanks @ktf