Support extra_target_triples targets from @rules_rust//tools/upstream_wrapper* for bazel_env

[aran]

@kolloch's bazel_env example is great. I tried to use it to use the Bazel-managed rust toolchain for non-Bazel-aware tools that invoke rustc, including a tool that directly invokes rustup and one that runs cargo. It would a big project to port those outside tools to Bazel, but it would still be a nice little win to consolidate on one Rust toolchain, which would avoid issues of syncing rustup and rust-toolchain.toml with the rules_rust configuration.

My wishlist feature request is to somehow support cross-compilation from @rules_rust//tools/upstream_wrapper:rustc, with theextra_target_triples in rust.toolchain taking the place of rustup target add.

In other words:

# MODULE.bazel
rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
rust.toolchain(
    edition = "2021",
    extra_target_triples = [
        "aarch64-unknown-linux-gnu",
        "wasm32-unknown-unknown",
        "x86_64-unknown-linux-gnu",
    ],
)

# .../BUILD.bazel
bazel_env(
    name = "bazel_env",
    # TODO: Upgrade when on Bazel 9+
    # toolchains = {
    #     "rust": "@rules_rust//rust/toolchain:current_rust_toolchain",
    # },
    tools = {
        # TODO: Uncomment when on Bazel 9+
        # "cargo": "$(CARGO)",
        # "rustfmt": "$(RUSTFMT)",
        # "rustc": "$(RUSTC)",
        # "rustdoc": "$(RUSTDOC)",
        "cargo": "@rules_rust//tools/upstream_wrapper:cargo",
        "cargo-clippy": "@rules_rust//tools/upstream_wrapper:cargo_clippy",
        "rustc": "@rules_rust//tools/upstream_wrapper:rustc",
        "rustfmt": "@rules_rust//tools/upstream_wrapper:rustfmt",

    },
...

Then:
wasm-pack build ...