crate_universe: Allow passing extra args down to crate_universe_bootstrap

[DolceTriade]

this is useful for rules_nixpkgs which allows us to specify an external rust toolchain controlled by Nix. Currently, we are required to carry a patch to modify rules_rust: https://github.com/tweag/rules_nixpkgs/tree/master/examples/toolchains/rust

Now we can add something like this to our WORKSPACE to use a custom cargo and rustc:

crate_universe_dependencies(
    bootstrap = True,
    rust_toolchain_cargo_template = "@nix_rust//:bin/{tool}",
    rust_toolchain_rustc_template = "@nix_rust//:bin/{tool}",
)

[UebelAndre]

Thanks! Just need to update docs by running docs/update_docs.sh and committing the results

[DolceTriade]

Done, thanks!