Use an absolute path for CompileShaders.cmd

[bghgary]

[Created by Copilot on behalf of @bghgary]

Same change as microsoft/DirectXTK#660, suggested by @walbourn there.

The shader build step names the script as a bare CompileShaders.cmd, which only resolves because Windows falls back to searching the current directory when looking up an executable. That fallback is disabled whenever NoDefaultCurrentDirectoryInExePath is present in the environment — its presence alone is enough, the value is irrelevant — so any hardened build or automation environment fails with Generating HLSL shaders... followed by no such file or directory.

WORKING_DIRECTORY already points at DirectXTex/Shaders, so naming the script by absolute path keeps the same intent without relying on current-directory lookup.

Verified on Windows with the Visual Studio 17 2022 generator, building the DirectXTex target:

	variable set	variable unset
before	fails: no such file or directory, error MSB8066	builds
after	builds	builds

The shader step regenerated all 14 .inc files in each passing case, and produced none in the failing case.

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