Multiple calls to `toolchain()` fails with bzlmod

It's currently not possible to have multiple calls to `toolchain()`, since under the hood it just calls `rust_register_toolchains` and the declared meta-repository names step on each other.

e.g.:
```bzl
rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
rust.toolchain(
edition = "2021",
versions = ["1.75.0"],
)
rust.toolchain(
edition = "2021",
versions = ["1.74.0-dev"],
sha256s = ...
)
```

My use-case is mixing-matching multiple toolchain versions and targets. This is doable without bzlmod by skipping `rust_register_toolchains` and instead calling `rust_repository_set` directly.