Update iohk-nix flake input from personal branch to main

## Problem The `iohkNix` flake input is pinned to commit `911835056d2b` which lives on branch `hkm/update-blst` — a personal branch, not the default branch. This is fragile: the branch could be deleted or rebased. ## Steps 1. Check if the blst changes from `hkm/update-blst` have been merged to the default branch 2. If merged: `nix flake update iohkNix` to pin to a main-branch commit 3. If not merged: document why the personal branch pin is needed 4. Build and test