Dont create build directory for zstd build

Author: Damir Porobic

.github/workflows/windows.yml

@@ -107,7 +107,6 @@ jobs:
         run: |
           git clone --depth 1 https://github.com/facebook/zstd.git
           cd zstd
-          mkdir build
           cd build && cmake .. -G"NMake Makefiles" -DCMAKE_INSTALL_PREFIX="$env:INSTALL_PREFIX" -DCMAKE_BUILD_TYPE=Release
           nmake
           nmake install