Pin Aeneus in lakefile.lean in SymCRust #59

Currently, aeneas is expected to be in a sibling directory and pinned to d71d2e3f.

However, aeneas can be explicitly pinned in the lakefile.lean by doing

require aeneas from git
  "https://github.com/AeneasVerif/aeneas" @ "d71d2e3f2cb763f7faf15ab606ac9cf32da8dead" / "backends/lean"

Disclosure: I am interested in using this repository to mine Lean4 proving challenges for evaluating and training LLMs, which I would like to publish to HuggingFace as open source. This change will make running these evaluations a little more turnkey.