Use different nixpkgs for rust and ocaml toolchains

[Nadrieril]

This should fix CI. The issue is as follows:

• we need recent rust-overlay versions to get access to recent nightlies;
• to be compatible with different ocaml versions, karamel, charon, aeneas and eurydice use pkgs.ocamlPackages as their ocaml toolchain instead of pinning a particular ocaml version;
• pkgs.ocamlPackages uses a different version of ocaml depending on the nixpkgs version;
• recent nixpkgs uses ocaml 5 by default, which fails to build some of our ocaml dependencies;
• recent rust-overlay is not compatible with old nixpkgs.

All this put together, the best solution I found was using a different nixpkgs for the rust and ocaml toolchains, implemented in charon in AeneasVerif/charon#522.

[Nadrieril]

This fails because it uses gcc instaed of clang to uild eurydice tests for some arcane reason. AeneasVerif/eurydice#136 should fix that.