Use PowerShell (pwsh) in windows step in main.yml (onnx#7311)

https://api.securityscorecards.dev/projects/github.com/onnx/onnx

complains about

"Info: Possibly incomplete results: error parsing shell code: statements
must be separated by &, ; or a newline: .github/workflows/main.yml:131",

---------

Signed-off-by: Andreas Fehlner <fehlner@arcor.de>

Author: andife

.github/workflows/main.yml

@@ -136,6 +136,7 @@ jobs:
           }
 
           workflow_scripts/protobuf/build_protobuf_win.ps1 -cmake_arch $cmake_arch
+        shell: pwsh
 
       - name: Build and install ONNX - Linux
         if: startsWith(matrix.os,'ubuntu')
@@ -187,13 +188,7 @@ jobs:
       - name: Install test dependencies
         run: |
           python -m pip install -r requirements-release_test.txt
-      
-      - name: Install Pillow with JPEG 2000 support
-        if: (matrix.python_version == '3.14-dev') && startsWith(matrix.os,'ubuntu')
-        run: |
-          sudo apt-get install -y libopenjp2-7-dev # TODO: integrate in test reqs
-          python -m pip install --upgrade Pillow --no-binary :all:
-          
+                
       - name: Run Python tests
         if: (matrix.python_version != '3.13t' ) && (matrix.python_version != '3.14-dev')
         run: |