[CI] vcpkg cache misses cause ~1h cold rebuilds

[patryk-przybysz]

Problem

Agent CI sometimes takes ~1 hour even when sources/agent/vcpkg.json is unchanged. Fast runs (~2 min) restore the cache; slow ones rebuild deps from source during CMake Configure (grpc alone ~49 min).

Cause

1. x-gha binary cache is gone - workflow sets VCPKG_BINARY_SOURCES=clear;x-gha,readwrite, but vcpkg removed that backend (~Apr 2025). Logs warn: "The 'x-gha' binary caching backend has been removed."
   • why run-vcpkg Is missing out on x-gha cache: it has been removed from vcpkg lukka/run-vcpkg#251
   • https://learn.microsoft.com/en-us/vcpkg/users/binarycaching
2. actions/cache misses the real binary cache dir - lukka/run-vcpkg puts packages under a per-run UUID path (VCPKG_DEFAULT_BINARY_CACHE), while we only cache ~/.cache/vcpkg + sources/agent/build/vcpkg_installed.
3. Eviction / branch scope - unused caches expire after 7 days; PR vs base/default branch visibility can miss the same key. Repo often ends up with 0 active caches after quiet periods.

Proposed fix

• Drop x-gha.
• Use a fixed VCPKG_DEFAULT_BINARY_CACHE and cache that with actions/cache.
• Optionally add NuGet → GitHub Packages for a durable shared binary cache across branches.

Refs

• Workflow: .github/workflows/agent_ci.yml
• Example slow run: https://github.com/monvit/volta/actions/runs/29690514046