Fix sccache cache empty due to SCCACHE_DIR configuration ordering

[Copilot]

sccache cache uploaded to GitHub Actions was empty because SCCACHE_DIR was configured after the cache restore/save step, causing a directory mismatch.

Changes

• Extract SCCACHE_DIR configuration into dedicated step before cache action
• Reorder workflow: Configure → Cache → Install → Build
• Ensures cache action operates on correct directory throughout job lifecycle

Before

- name: Install Dependencies
  run: |
    echo "SCCACHE_DIR=$GITHUB_WORKSPACE/.sccache" >> $GITHUB_ENV  # Too late
    mkdir -p .sccache
    
- name: Cache sccache  # Runs with wrong/unset SCCACHE_DIR
  uses: actions/cache@v4

After

- name: Configure sccache
  run: |
    echo "SCCACHE_DIR=$GITHUB_WORKSPACE/.sccache" >> $GITHUB_ENV
    mkdir -p .sccache
    
- name: Cache sccache  # Now uses correct SCCACHE_DIR
  uses: actions/cache@v4

The cache action now restores to and saves from the same directory that sccache writes to during compilation.

Original prompt

Benchx CLI Publish的sscache缓存没有生效，上传到github actions/cache 的缓存为空，帮我看看为啥

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