CG2 test followups

[am11]

https://github.com/dotnet/runtime/pull/80154 followups:

• [ ] __TestBuildMode: crossgen2 (as in git grep __TestBuildMode.*crossgen2) should use helix so we can revert https://github.com/dotnet/runtime/commit/232460256bfc7712105e1382db2b5e7fea372afd (i.e. use published crossgen2.exe instead of dotnet crossgen2.dll)

• [ ] Crossgen2Tool (as in git grep '<Crossgen2Tool' should similarly use published crossgen2.exe instead of crossgen2.dll and while working on it, we should deduplicate

  runtime/src/tasks/Crossgen2Tasks/PrepareForReadyToRunCompilation.cs
  runtime/src/tasks/Crossgen2Tasks/ResolveReadyToRunCompilers.cs
  runtime/src/tasks/Crossgen2Tasks/RunReadyToRunCompiler.cs

  and

  sdk/src/Tasks/Microsoft.NET.Build.Tasks/PrepareForReadyToRunCompilation.cs
  sdk/src/Tasks/Microsoft.NET.Build.Tasks/ResolveReadyToRunCompilers.cs
  sdk/src/Tasks/Microsoft.NET.Build.Tasks/RunReadyToRunCompiler.cs

• [ ] Remove apphost.exe requirement from crossgen2_publish; some ResolveFrameworkRef.. type of target make it mandatory during the publish. It will allow us to delete this two-step workaround https://github.com/dotnet/runtime/blob/62835af00823d0f4d4138744dec90e65579f2e69/src/tests/Common/Directory.Build.targets#L59-L64
• [ ] Integrate src/tests/build.sh/cmd with top-level build.sh/cmd, so when we use build.sh -c release -rc checked, the corresponding test command is src/tests/build.sh -release -p:RuntimeConfiguration=checked instead of what we have now src/tests/build.sh -checked -p:LibrariesConfiguration=release. This will allow us to remove workaround like https://github.com/dotnet/runtime/blob/62835af00823d0f4d4138744dec90e65579f2e69/src/tests/Common/Directory.Build.targets#L40-L57

[steveisok]

/cc @ivdiazsa