[actionpins] Make per-repo pin sorting mutation explicit in buildByRepoIndex

[Copilot]

This change addresses the fp-enhancer update for pkg/actionpins by making the per-repository sort operation explicit without changing behavior. The previous implementation was correct but relied on implicit slice backing-array mutation through a ranged value copy.

• Problem scope

  • Touches only pkg/actionpins index construction logic used for action pin resolution.
  • Removes ambiguity in how sorted results are applied back to byRepo.

• Code change

  • In buildByRepoIndex, replaced map range-by-value sorting with key-based access:
    • from sorting repoPins (range value)
    • to sorting byRepo[key] directly

• Why this matters

  • Makes in-place mutation intent unambiguous to readers.
  • Reduces risk of future “defensive” rewrites (e.g., redundant re-assignment) around map/slice semantics.

// before
for _, repoPins := range byRepo {
    slices.SortFunc(repoPins, cmpByVersionDesc)
}

// after
for key := range byRepo {
    slices.SortFunc(byRepo[key], cmpByVersionDesc)
}