Scope SDK-generated MSBuild paths to the current invocation

[OvesN]

Problem

dotnet sets MSBuildExtensionsPath and MSBuildSDKsPath for its selected SDK. Descendant dotnet commands inherit those values even when they select a different SDK, which can mix SDK components and fail while loading tasks.

Failure: https://github.com/jaredpar/complog/actions/runs/31601500191/job/94129782284?pr=375
Repro/details: #14699

Expected

SDK-generated paths should be scoped to the immediate MSBuild invocation, not exposed as ambient state to unrelated child processes. Explicit user overrides must remain supported.

The top-level CLI should decide the effective values and map them to MSBuild through CLI-owned startup state/properties; nested dotnet commands should compute paths for their own selected SDK.

Design proposal

1. At CLI entry, read any explicit user overrides.
2. Otherwise compute paths from the SDK selected by that invocation.
3. Pass the effective paths only to the immediate MSBuild instance through CLI-owned startup state or global properties, rather than setting generated defaults in the ambient environment.
4. Propagate the values explicitly to MSBuild worker nodes.

This preserves intentional user overrides while allowing each nested dotnet invocation to use a coherent SDK.