ci: align registry validation with Go 1.26.4

[intel352]

Summary

• Align registry validation Go setup and cache keys with Go 1.26.4
• Keep wfctl core registry validation compatible with the current workflow module toolchain

Root cause

Validate core manifests against workflow plugins checks out GoCodeAlone/workflow, then runs wfctl plugin registry-sync core under GOTOOLCHAIN=local. The workflow module now declares Go 1.26.4, but registry validation was still provisioning Go 1.26.3, causing the inspect build to fail with only exit status 1.

Verification

• actionlint .github/workflows/*.yml

[Copilot]

Pull request overview

Updates the CI registry validation workflow to provision the same Go patch version as the GoCodeAlone/workflow module toolchain, preventing wfctl plugin registry-sync core from failing under GOTOOLCHAIN=local due to a Go version mismatch.

Changes:

• Bump actions/setup-go go-version from 1.26.3 to 1.26.4 for core-manifest validation.
• Update Go module cache key + restore prefix to match 1.26.4, keeping cache segregation consistent with the provisioned toolchain.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.