Update trimming tests to use live-built ILLink.Targets logic

[eerhardt]

The trimming tests are not using the current ILLink.Targets logic that comes from the dotnet/runtime repo. This is a left-over of the previous versions when this logic came from the dotnet/linker repo. But it is now contained in dotnet/runtime, so we should be using the current repo version so PRs are validated with the updated logic.

          My understanding is that the `trimmingTests` failing here are using the package version picked up in this PR, based on

runtime/eng/testing/linker/project.csproj.template

Lines 55 to 62 in a41f655

	<!-- Logic to override the default IlLink tasks that come from the SDK and use the one
	we use in dotnet/runtime repo -->
	<ItemGroup>
	<PackageReference Include="Microsoft.NET.ILLink.Tasks"
	Version="{MicrosoftNETILLinkTasksVersion}"
	PrivateAssets="all"
	IsImplicitlyDefined="true" />
	</ItemGroup>

Originally posted by @sbomer in #88245 (comment)

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Issue Details

---

The trimming tests are not using the current ILLink.Targets logic that comes from the dotnet/runtime repo. This is a left-over of the previous versions when this logic came from the dotnet/linker repo. But it is now contained in dotnet/runtime, so we should be using the current repo version so PRs are validated with the updated logic.

          My understanding is that the `trimmingTests` failing here are using the package version picked up in this PR, based on

runtime/eng/testing/linker/project.csproj.template

Lines 55 to 62 in a41f655

	<!-- Logic to override the default IlLink tasks that come from the SDK and use the one
	we use in dotnet/runtime repo -->
	<ItemGroup>
	<PackageReference Include="Microsoft.NET.ILLink.Tasks"
	Version="{MicrosoftNETILLinkTasksVersion}"
	PrivateAssets="all"
	IsImplicitlyDefined="true" />
	</ItemGroup>

Originally posted by @sbomer in #88245 (comment)

Author:	eerhardt
Assignees:	-
Labels:	untriaged, area-Tools-ILLink
Milestone:	-

[radical]

cc @lewing