infra[notask]: use setup-vcpkg action in cpp test workflows

[tamer-hassan-tether]

Summary

• Replace duplicated per-OS vcpkg clone/bootstrap steps in cpp test workflows with the shared setup-vcpkg composite action
• Unify vcpkg cache directory creation across platforms using bash mkdir -p
• Remove the obsolete Windows VCPKG_INSTALLATION_ROOT override from reusable-prebuilds.yml

Test plan

• Trigger cpp test workflows (llm, embed, diffusion, vla) on Linux, macOS, and Windows and confirm vcpkg setup succeeds
• Confirm reusable prebuilds still bootstrap vcpkg correctly on Windows

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ❌ (0/1)
- 1 Team Lead OR Management approval ✅ (2/1)

**Bypass rule:** Triggered (2+ Team Lead approvals (Tier 1 exception)). This PR is approved regardless of tier.

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*This comment is automatically updated when reviews change.*