fix: restore PR head branch detection lost in #172 merge

[Kalindi-Dev]

Summary

Re-apply the GH_HEAD_REF fix from #168 which was accidentally overwritten when #172 was merged. Without this, every PR-triggered CodeBuild run fails at Stage 1 because CURRENT_BRANCH resolves to 155/merge instead of the actual PR source branch.

Changes

• Re-add GH_HEAD_REF: ${{ github.head_ref }} to the buildspec env variables
• Restore the fallback chain: ${GH_HEAD_REF:-${GH_REF_NAME:-}}

Checklist

• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Test Plan

• Trigger a CodeBuild run from a PR and verify CURRENT_BRANCH resolves to the PR head branch

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[scottschreckengaust]

LGTM

[harmjeff]

Approved