[CI] Fix Rust Docs Build

[tqchen]

see #7743

The recent outrages coulde due to multiple folds of reasons:

• The lack of environment variable updates as in task_rust.sh (LLVM_CONFIG_PATH) vs the task_python_docs
• Change of cargo behavior due to updates? (more likely below)
• The cargo might caches previous docs build if files does not change (the recent PR of Device refactor triggers a rebuild).

Possible steps:

• Fix the rust docs build script to make sure it always build correctly
• Consider some option like force rebuild that always rebuilds the docs(assuming not taking a lot time) regardless of caching

cc @Mousius @areusch @driazati @gigiblender

[binarybana]

I believe the key error was:

error: couldn't read /workspace/rust/tvm-sys/src/c_runtime_api.rs: No such file or directory (os error 2)

And not the LLVM path because that LLVM_CONFIG_PATH warning shows up in previous successful runs.

c_runtime_api.rs is normally generated by bindgen here. So perhaps that got deleted and bindgen didn't get triggered as part of a semi-stale cache (so didn't trigger build.rs)? But I don't have time to dig deeper.

[tqchen]

cc @jroesch