chore(merkle-trees): pin compiler_version

[sripwoud]

nargo version is already pinned in the GH action workflow (see privacy-scaling-explorations/zk-kit#256).
But not in the Nargo.toml.

With the latest version (that I was using) the tests will fail because of breaking changes in the std lib:
ex

error: Could not resolve 'HashPath' in path
   ┌─ /home/sripwoud/code/zk-kit/zk-kit.noir/packages/merkle-trees/src/merkle.nr:25:22
   │
25 │ impl Modifier<Field, HashPath> for MerkleTree {
   │                      --------
   │

error: cannot find `pedersen_hash_slice` in this scope
  ┌─ /home/sripwoud/code/zk-kit/zk-kit.noir/packages/merkle-trees/src/merkle/tests/pedersen.nr:5:5
  │
5 │     pedersen_hash_slice(leaves)
  │     ------------------- not found in this scope

[signorecello]

Thanks, yes best to pin versions