fix: panic when building bitcoin state in bootstrap script

[ielashi]

When trying to build the bitcoin state from the bootstrap scripts, the last step (./6_compute_canister_state.sh) fails with the following:

Shuffling...
Writing to stable structure...
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', bootstrap/state-builder/src/build_balances.rs:97:49
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is because, previously insert on StableBTreeMap returned a result. Currently, insert doesn't fail and that unwrap is no longer needed.

[github-actions]

canbench 🏋

No significant performance changes detected ✅

./canbench_results.yml is up to date ✅

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 561.77 M (-0.00%) (change within noise threshold)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: get_metrics
  total:
    instructions: 87.00 M (-0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (-0.00%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (-0.00%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------