ci : build Windows TurboQuant in a short directory

[danyurkin]

Overview

Fixes the windows-x64 (vulkan) job, which fails on the current windows-2022 runner image (20260913.307.1). Because of that failure, the Windows Vulkan zip is missing from turboquant-29d8598.

The failure comes from the vulkan-shaders-gen sub-build, which cannot pass CMake's compiler check:

CMakeFiles\cmTC_xxxxx.dir\Debug/manifest.rc(3) : error RC2136 : missing '=' in EXSTYLE=<flags>
Check for working C compiler: ... cl.exe - broken

• The same code built on image 20260802.262.1 (run for turboquant-07b9908).
• On the new image, the top-level configure and the cpu / CUDA variants pass. Only the sub-build fails.
• This repo's name appears twice in the workspace path (D:\a\atomic-llama-cpp-turboquant-nightly\atomic-llama-cpp-turboquant-nightly, 76 chars). The sub-build's try_compile directory sits deep under build\ggml\src\ggml-vulkan\vulkan-shaders-gen-prefix\....

Change

• Build in C:\tqb instead of .\build, then add a junction from build to C:\tqb. The later steps (verify, CUDA DLL bundling, signing, packaging, release) still use .\build\bin\Release and are unchanged.
• Add || exit /b 1 to both build commands. The cmd step returns the exit code of its last command, which is now mklink, so without this a failed build would pass.

Testing

I ran it on the fork with the same runner image (20260913.307.1), CMake 3.31.6 and Windows SDK 10.0.26100 rc.exe:

• Unchanged workflow, build dir: fails in 1 min with the same RC2136.
• Same job with C:\tqb: builds (ggml-vulkan.dll, 53 MB).
• This workflow file, vulkan variant only, with signing and release removed because the fork has no secrets: every step passes. The log shows Junction created for build <<===>> C:\tqb, llama-server.exe --version runs from .\build\bin\Release, and the packaged llama-turboquant-windows-x64-vulkan.zip is 27 MB, the same as the previous release.

Additional information

Merging this PR changes a build-turboquant-*.yml file, so the shared paths filter from #2 runs all four platforms on the merge commit. That produces a complete new release that includes the Windows Vulkan zip, so there is no need to re-run the failed job on 29d8598.