ci (vs-build): use cmd to copy the DLLs, not powershell

We use a `.bat` script to copy the DLLs in the `vs-build` job, and those
type of scripts are native to CMD, not to PowerShell.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/main.yml

@@ -147,10 +147,8 @@ jobs:
     - name: add msbuild to PATH
       uses: microsoft/setup-msbuild@v1
     - name: copy dlls to root
-      shell: powershell
-      run: |
-        & compat\vcbuild\vcpkg_copy_dlls.bat release
-        if (!$?) { exit(1) }
+      shell: cmd
+      run: compat\vcbuild\vcpkg_copy_dlls.bat release
     - name: generate Visual Studio solution
       shell: bash
       run: |