Envoy 1.32 has added some more dependencies on rules_rust bits, so we
need to support that now.
In particular, for some reason, we always fail with a message about
needing repinning, so we repin during the IFD phase, but we need to
point rules_rust at the nixpkgs Rust toolchain, make it build the
cargo-bazel tool (because the one in nixpkgs is the wrong version), and
then clean up afterwards.
