rust: Fix build break because of download-ci-llvm

rust: Fix build break because of download-ci-llvm

Previously, download-ci-llvm was set to false. However, with the following commit:
https://github.com/rust-lang/rust/commit/7d579046c80d3de3143dcb8b2db5640f95b5383c ,
which has been present from rust_1.83, it was changed to true. As a result, after
updating to rust_1.83, we encountered the following error during the build:

-------------------------------------------------------------------------------
| thread 'main' panicked at src/core/config/config.rs:2047:13:
| setting build-target.llvm-config is incompatible with download-ci-llvm.
| note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
-------------------------------------------------------------------------------

To resolve this issue, we are setting download-ci-llvm back to false.

Signed-off-by: Deepesh Varatharajan <Deepesh.Varatharajan@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>