Fix cache ordering in CI workflow to avoid redundant downloads

[Copilot]

The cache restore step was positioned after the download step, causing the download to execute on every run and potentially being overwritten by the restored cache.

Changes:

• Reordered workflow steps: cache restore now precedes download
• Download only executes on cache miss when ./src/GRCh37.tar.gz doesn't exist
• Cache save occurs automatically at job completion

Before:

- name: Download GRCh37.tar.gz if not present
  # Always runs, then gets overwritten by cache
- name: Cache src directory
  # Restores after download

After:

- name: Cache src directory
  # Restores first if available
- name: Download GRCh37.tar.gz if not present
  # Only runs on cache miss

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