Add GitHub Actions artifact upload support for issue reports

[Copilot]

Implements artifact upload functionality for GitHub Actions builds, enabling issue reports to be uploaded as build artifacts similar to Azure DevOps and AppVeyor support.

Changes Made

Core Implementation

• GitHubActionsBuildServer.cake: Updated PublishIssuesArtifacts method to set GitHub Actions output variables
• GitHubActionsBuildServer.cs: Mirror implementation for Cake.Frosting variant with proper imports

Features

✅ Modern GitHub Actions Support: Uses $GITHUB_OUTPUT environment file when available
✅ Backward Compatibility: Falls back to ::set-output workflow commands for older runners
✅ Configuration Respect: Honors ShouldPublishFullIssuesReport and ShouldPublishSarifReport parameters
✅ File Validation: Only sets outputs when report files actually exist

How It Works

The implementation sets GitHub Actions output variables that workflows can consume:

- name: Run Build with Issues
  id: build
  run: dotnet cake --target=Publish-IssuesArtifacts

- name: Upload Issues Report
  if: steps.build.outputs.full-issues-report-path
  uses: actions/upload-artifact@v4
  with:
    name: Issues Report
    path: ${{ steps.build.outputs.full-issues-report-path }}

- name: Upload SARIF Report  
  if: steps.build.outputs.sarif-report-path
  uses: actions/upload-artifact@v4
  with:
    name: SARIF Report
    path: ${{ steps.build.outputs.sarif-report-path }}

Output Variables

• full-issues-report-path: Path to HTML issues report
• sarif-report-path: Path to SARIF report

Testing

Comprehensive testing performed including:

• Normal operation with both report types
• Fallback mode compatibility
• Edge cases (missing files, disabled publishing)
• Syntax validation and compilation

This brings GitHub Actions support in line with existing Azure DevOps and AppVeyor artifact upload capabilities.

Fixes #376.

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