Pin Rust toolchain, use Rustls

[calyptobai]

Based on our flake.nix file, it seems we have the following runtime dependencies: openssl.out rocksdb git zlib nsync onnxruntime14. Judging from our own Cargo.lock, and examining qdrant's Cargo.lock, we know that:

• rocksdb is built statically and not needed at runtime (unless provided by environment variables, which we don't do in default builds)
• zlib is not used as default features are deactivated
• We removed libgit2 in favor of gitoxide
• We pin onnxruntime versions to specific binaries provided by ort

We also add a Rust toolchain configuration to pin the toolchain at 1.70.0, and ensure that openssl-sys is built statically using the vendored feature.

This removes all remaining non-platform-specific runtime dependencies except nsync, which does not seem to be linked into either bloop builds or qdrant's builds.