ci: use Node 24 cache action for registry Go jobs

[intel352]

Summary

• disable setup-go built-in caching so it no longer invokes the hidden Node 20 actions/cache@v4 path
• add explicit actions/cache@v5.0.5 pinned by SHA for Go module/build cache
• keep the existing wfctl registry validation and sync behavior unchanged

Verification

• bash scripts/validate-manifests.sh
• bash tests/test-build-index.sh
• bash tests/test-schema-allowlist-coverage.sh
• bash tests/test-sync-workflow-token.sh
• bash tests/test-build-pages-release-token.sh
• actionlint .github/workflows/*.yml
• git diff --check

[Copilot]

Pull request overview

This PR updates the registry’s GitHub Actions workflows to avoid actions/setup-go’s implicit cache path (which uses an older Node runtime) and instead uses an explicit, pinned actions/cache@v5 (Node 24) step for Go build/module caching, while leaving the existing wfctl validation/sync behavior intact.

Changes:

• Disable actions/setup-go built-in caching (cache: false) for the affected Go jobs.
• Add an explicit Go cache step using actions/cache@v5.0.5 pinned by SHA, caching ~/.cache/go-build and ~/go/pkg/mod.
• Keep existing checkout / wfctl steps and behavior unchanged, including the plugin-filtered sync flow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/validate.yml	Replaces setup-go caching with an explicit actions/cache@v5.0.5 step for the core-manifest validation Go job.
.github/workflows/sync-registry-manifests.yml	Replaces setup-go caching with an explicit actions/cache@v5.0.5 step for the Go-dependent sync path, preserving existing conditional execution.

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