ci: enable vcpkg binary caching in main_ci

[alandtse]

Why

The windows build matrix recompiles vcpkg dependencies from source on every run. VCPKG_DEFAULT_BINARY_CACHE points at ${{ github.workspace }}/vcpkg-cache — a workspace-local dir that's wiped each run, so it never actually persisted anything.

What

Switch to the GitHub Actions cache as a vcpkg binary source:

• VCPKG_BINARY_SOURCES: "clear;x-gha,readwrite" (replaces the dead local-dir cache).
• A github-script step re-exports ACTIONS_CACHE_URL / ACTIONS_RUNTIME_TOKEN (GitHub only exposes these to the github-script runtime; vcpkg's x-gha provider needs them).

Now dependency .libs are restored across runs and matrix configs instead of recompiled. Validated locally on a sibling repo: a warm cache turns a dep build into a ~ms restore.

Part of a stack-wide sweep (CommonLibVR, CrashLoggerSSE, Buffout4, EngineFixesSkyrim64 already done).

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6f3e18fe-7ece-4cb7-9db8-01551d409b10

📥 Commits

Reviewing files that changed from the base of the PR and between 0a0a8df and 5f0855b.

📒 Files selected for processing (1)

• .github/workflows/main_ci.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/vcpkg-binary-cache

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