Create a pipeline that tests sdk tasks based on existing sdk tests.

## Context

SDK has end-to-end tests that exercise the full `dotnet build` / `dotnet publish` / etc. pipeline. These tests currently run only in single-threaded mode. To validate the MT task-enlightenment work, the team needs these tests running with multi-threaded mode enabled.

## Goal

Create a pipeline (or pipeline variant) in the SDK repo that runs existing SDK end-to-end tests with the multi-threaded MSBuild mode enabled via environment variable.

## Approach

- Identify the SDK test utility methods that invoke `dotnet build` and similar commands.
- Flow an environment variable or property to enable MT mode through those invocations.
- Set up a separate pipeline (or pipeline leg) that enables the MT flag for the full SDK test suite.
- The pipeline configuration may need to pass the MT attribute at the pipeline-definition layer, or it may require a code change in the SDK test infrastructure to read an environment variable and apply it.

## Exit Criteria

- A pipeline exists that runs SDK end-to-end tests with multi-threaded MSBuild enabled.
- Results are visible and can be monitored for regressions introduced by MT mode.

## Notes

- This is a single pipeline (not a matrix across all platforms).
- Discussed in grooming on June 10, 2026.