`download-ci-llvm = true` is set in rustc config.toml, but I get `No such file or directory` error


changelog-seen = 2

# Whether to use Rust CI built LLVM instead of locally building it.
download-ci-llvm = true     # Download a pre-built LLVM?
assertions = true           # LLVM assertions on?
ccache = "/usr/bin/ccache"  # Use ccache when building LLVM?

debug-logging = true        # Leave debug! and trace! calls in rustc?
incremental = true          # Build rustc with incremental compilation?

Run Command on ubuntu 20.04

$ RUST_BACKTRACE=1 ./x.py build -i library/std                                                                                                         
Updating only changed submodules
  Submodules updated in 0.02 seconds
Building rustbuild
    Finished dev [unoptimized] target(s) in 0.41s
thread 'main' panicked at 'std::fs::read_to_string(ci_llvm.join("link-type.txt")) 
failed with No such file or directory (os error 2) 
("CI llvm missing: /rust/rust/build/x86_64-unknown-linux-gnu/ci-llvm")', src/bootstrap/config.rs:1010:37
stack backtrace:
   0: rust_begin_unwind
             at /rustc/0f231250ba7fe3a3f98a18aee9abfd65b104fb92/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/0f231250ba7fe3a3f98a18aee9abfd65b104fb92/library/core/src/panicking.rs:143:14
   2: bootstrap::config::Config::parse
             at ./src/bootstrap/config.rs:1010:37
   3: bootstrap::main
             at ./src/bootstrap/bin/main.rs:14:18
   4: core::ops::function::FnOnce::call_once
             at /rustc/0f231250ba7fe3a3f98a18aee9abfd65b104fb92/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I'm not sure what I'm missing.