Cargo is not a hermetic build system.
If some dependencies are removed from the system and provided them via Bazel, we need to tell cargo where to find them.
We do this by creating a .envrc.cargo file with paths to the Bazel-provided dependencies.
Combined with https://github.com/buildbuddy-io/bazel_env.bzl, this allows cargo to find dependencies provided by Bazel.
See the examples for a usage example.