Fix VS target hook points in Arcade SDK

[ViktorHofer]

Regressed with ea5fa16

Use standard hook points that are called by Build.proj (when invoking AfterSigning.proj) instead of these cutom ones.

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[Copilot]

Pull request overview

This PR fixes a regression in the Arcade SDK Visual Studio insertion pipeline by switching MSBuild target hook points to ones that are actually invoked by Build.proj during the AfterSigning.proj phase.

Changes:

• Run GenerateVisualStudioInsertionManifests before Pack (instead of before AfterPack) so it triggers in the AfterSigning.proj phase.
• Run _BuildTrainingInputs before Pack (instead of before AfterBuild) so IBC training inputs are generated during the AfterSigning.proj phase.

Show a summary per file

File	Description
src/Microsoft.DotNet.Arcade.Sdk/toolset/VisualStudio.InsertionManifests.targets	Updates manifest generation to hook into Pack, ensuring it runs when AfterSigning.proj is invoked by Build.proj.
src/Microsoft.DotNet.Arcade.Sdk/toolset/VisualStudio.BuildIbcTrainingInputs.targets	Updates IBC training input generation to hook into Pack, ensuring it runs in the expected AfterSigning.proj execution flow.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0