Instead of keeping a copy of analysis, we can fetch it directly from the upstream git repository.
Note that to improve network use, a shallow copy is explicitly used (see .cargo/config.toml).
The latest version of analysis (with a small change to the Cargo.toml) makes cargo a bit buggy due to a weird dependency hole, which may be fixed in a newer version of cargo.
Furthermore, due to the cargo configuration of prusti, cargo must be configured with the -Zbindeps flags (see flake.nix).
