CI: install net46 developer pack for NuGet pack builds

[Copilot]

The build workflow was failing in Nuget pack because project builds during packing require .NETFramework v4.6 reference assemblies, which were not present on the runner. This updates the workflow dependency to match the actual target framework used by the API projects.

• Root cause

  • nuget pack -build triggers MSBuild for net46 projects (AIAPI, HS2API, ECAPI, KKSAPI).
  • Workflow installed netfx-4.6.1-devpack, but the failure was MSB3644 for .NETFramework,Version=v4.6.

• Change

  • Updated CI workflow to install the correct targeting pack:

  - name: Install .NET Framework 4.6 targeting pack
    shell: pwsh
    run: |
      choco install netfx-4.6-devpack -y

• Impact

  • Aligns runner toolchain with project target frameworks so nuget pack -build can resolve net46 reference assemblies.