CI pipelines should use repo build scripts instead of UseDotNet@2 #706

Problem

Both the PR and official CI pipelines use the UseDotNet@2 Azure DevOps task with version: 8.x to install the .NET SDK, bypassing the repo's own build infrastructure (eng/common/build.sh, eng/common/CIBuild.cmd).

This causes several issues:

1. SDK version drift — The pipelines install whatever the latest 8.0.x SDK is (currently 8.0.420), which may differ from the version specified in global.json (8.0.126). When upgrading the SDK, you have to update both global.json and the pipeline YAML, rather than just one place.

2. Inconsistency with local development — Developers building locally use the build scripts which honor global.json, while CI uses a potentially different SDK version. This can mask or introduce issues that don't reproduce in the other environment.

3. Maintenance burden — The UseDotNet@2 version must be manually kept in sync across 6 pipeline locations (3 jobs × 2 pipeline files). The build scripts already handle SDK acquisition correctly from a single source of truth.

Suggestion

Replace the UseDotNet@2 task and separate DotNetCoreCLI@2 test task with a single call to the repo's build scripts (eng/common/build.sh / eng/common/CIBuild.cmd). The build scripts handle SDK installation automatically (honoring global.json) and then build/test — consolidating two pipeline steps into one and eliminating the version drift problem.