Keep Publish* properties across Restore so that we can gather all required implicit packages

[baronfel]

Fixes #49493

Before, when the implicit Restore was triggered for the packages we would unconditionally set the Publish* properties to false. This caused Restore to not add the implicit packages to the trimming tools, so later when the ILLink target was called we'd invoke the dummy one from the SDK, not the actual one from the ILLink targets package.

Added a test that checks the trimmed package contents and ensures that a dll whose functionality we never use is not in the package. We could also do a size-based check here - the trimmed packages are currently ~9MB vs the full packages which are ~25MB.

[Copilot]

Pull Request Overview

This PR preserves publishing-related MSBuild properties during restore to ensure implicit packages (like trimming tools) are correctly included and adds tests to verify trimmed tool packages exclude unused DLLs.

• Introduce a Trimmed flag in TestToolBuilder and update identifier logic
• Add an end-to-end test for trimmed tools checking package contents
• Update Microsoft.NET.PackTool.targets to guard publish properties by MSBuildIsRestoring

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
test/Microsoft.DotNet.PackageInstall.Tests/TestToolBuilder.cs	Add Trimmed setting and adjust identifier and project properties
test/Microsoft.DotNet.PackageInstall.Tests/EndToEndToolTests.cs	Add PackagesMultipleTrimmedToolsWithASingleInvocation test
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.PackTool.targets	Preserve publish properties when restoring by checking MSBuildIsRestoring

Comments suppressed due to low confidence (1)

test/Microsoft.DotNet.PackageInstall.Tests/EndToEndToolTests.cs:211

• The XML doc refers to 'dependency' but the method checks for file entries; consider updating the summary to clarify it checks for absent files in the package.

        /// Opens the nupkg and verifies that it does not contain a dependency on the given dll.

[marcpopMSFT]

It's possible that SelfContained doesn't matter here but the rest do and it's related to the inferences we make earlier in evaluation.