[ci] update apt index before clang install

[zhangxp1998]

Summary

• run apt-get update before installing clang/lld in the Clang CI workflow
• apply the same setup fix to the Rust workflow, which installs clang/lld the same way

Motivation

Several clang-20 jobs recently failed before reaching the build step because the runner's apt package index pointed at stale package versions that returned 404 from Ubuntu mirrors. Refreshing the package index before installing clang/lld makes this setup step less flaky on GitHub-hosted runners.

Test

• git diff --check

[Copilot]

Pull request overview

This PR reduces CI flakiness on GitHub-hosted Ubuntu runners by refreshing the apt package index before installing versioned clang/lld in workflows that depend on Ubuntu’s apt repositories.

Changes:

• Add sudo apt-get update before installing clang-${{ matrix.toolchain-ver }} and lld-${{ matrix.toolchain-ver }} in the Clang CI workflow.
• Apply the same apt index refresh in the Rust CI workflow, which installs the same toolchain packages.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/github-ci-rust.yml	Updates apt package index before installing clang/lld in the toolchain setup step.
.github/workflows/github-ci-clang.yml	Updates apt package index before installing clang/lld in the toolchain setup step.

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