Reenable Crossgen Tests

### Summary
The current crossgen tests in the `dotnet/sdk` repository use the `[Fact(Skip = "This coverage needs to be in core-sdk, which is where crossgen is applied")]` attribute. This indicates that the coverage for these tests should be in the `core-sdk` repository, where crossgen is applied.

Specifically, the following two tests are skipped:

1. [CLI_SDK_assemblies_must_be_crossgened](https://github.com/dotnet/sdk/blob/f6365c6aa5038215608694ec336b99b0e6377aca/test/crossgen.Tests/crossgen.Tests.cs#L18)
2. [Shared_Fx_assemblies_must_be_crossgened](https://github.com/dotnet/sdk/blob/f6365c6aa5038215608694ec336b99b0e6377aca/test/crossgen.Tests/crossgen.Tests.cs#L28)

The reason for skipping these tests is that the crossgen process (precompiling assemblies to improve startup performance) is handled in the `core-sdk` repository. Therefore, tests related to crossgen should ideally be located in `core-sdk`. However, to improve traceability and provide context, we propose replacing the Skip attribute with a link to this PR. This change will help developers understand the reason for skipping these tests.

### Expectation
We expect these two tests to be reenabled and properly executed within the appropriate context.