Fix D3D12Core.dll loading problem for debug build

[jkwak-work]

D3D12Core.dll had been copied to a wrong directory and slang has been using D3D12Core.dll from the system directory, C:\windows\system32.

D3D12Core.dll has to be copied from external/slang-binaries/bin/windows-x64 to build/Release/bin/D3D12 not to build/Release/bin.
The same is true for the debug build and it had to be copied to build/Debug/bin/D3D12 not build/Debug/bin.

It hasn't been a problem for Release build, because the debug-layer is not enabled for Release build and it didn't cause the version mismatching problem with D3D12SDKLayers.dll. The Release build was loaded from either build/Release/bin or from C:\windows\system32, and it didn't matter which one was used.

The Debug build, however, got into a problem where D3D12Core.dll was loaded from the system directory whereas D3D12SDKLayers.dll was loaded from build/Debug/bin and it failed to load D3D12.dll entirely. This caused D3D12 to be "Not supported" for "Windows/Debug" configuration. Note that our CI explicitly excludes DX12 tests for the "Windows/Debug" configuration with a command-line argument "-api all-dx12", and DX12 tests were going to be ignored anyway.

The actual problem was observed when WGPU is implemented. WGPU started printing explicit errors for the load failure of D3D12.dll.

See more detailed explanation:
https://devblogs.microsoft.com/directx/gettingstarted-dx12agility/#d3d12sdkpath-should-not-be-the-same-directory-as-the-application-exe

Closes #5305
Closes #5276

[jkwak-work]

With this PR, the "Windows/Debug" test in CI shows the following,

Compiling stdlib on debug build, this can take a while.
Compiling stdlib took 21.00 seconds.
Supported backends: fxc dxc glslang spirv-dis clang visualstudio gcc genericcpp llvm spirv-opt tint
Check dx12,d3d12: Supported
Check dx11,d3d11: Supported
Check cuda: Not Supported

Note that D3D12 is supported for Debug build.

Without this PR, the previous output shows that D3D12 was not supported for Debug build

Compiling stdlib on debug build, this can take a while.
Compiling stdlib took 21.00 seconds.
Supported backends: fxc dxc glslang spirv-dis clang visualstudio gcc genericcpp llvm spirv-opt tint
Check dx12,d3d12: Not Supported
Check dx11,d3d11: Supported
Check cuda: Not Supported
Warning: D3D12CreateDevice failed
 - While initializing adapter (backend=BackendType::D3D12)
    at InitializeImpl (D:\a\webgpu-dawn-binaries\webgpu-dawn-binaries\build\_deps\dawn-src\src\dawn\native\d3d12\PhysicalDeviceD3D12.cpp:[93](https://github.com/shader-slang/slang/actions/runs/11354984008/job/31583392799#step:5:94))

[aleino-nv]

LGTM but can the test be disabled instead of commenting out the assert?
The assert seems to be just because of some file extension specific logic, and should otherwise hold.

There will be another PR that addresses this problem properly.