bindgen: don't `register_toolchains` in Bzlmod (#3231)

I'm currently in the process of getting rules_ros2 onto Bzlmod. One
challenge is that its rust support requires rules_rust_bindgen, which
depends on llvm-project, which takes more than GitHub Action's maximum
configurable job timeout (10h) to build. See
https://github.com/mvukov/rules_ros2/pull/238#discussion_r1929687153

This PR adds new "bring your own clang" build flags:
`--@rules_rust_bindgen//:clang` and `--@rules_rust_bindgen//:libclang`.
As the added test shows, you can point these at toolchains_llvm's
pre-compiled clang.

The fancy !!set and << merge key yaml magic is documented in
https://github.com/bazelbuild/continuous-integration/tree/master?tab=readme-ov-file#sharing-configuration-between-tasks

Let me know what you think about this. I'm also curious what's the
rationale for depending on llvm-project instead of toolchains_llvm in
the first place. Maybe instead of making this configurable (and thus
adding complexity) rules_rust_bindgen could just be switched over to
toolchains_llvm completely?

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Co-authored-by: UebelAndre <github@uebelandre.com>