(build) Specify GHA shell as Windows PowerShell

The default shell on Windows runners is pwsh, but running the PSSA
task under pwsh was resulting in an error running `Install-Module`.
This error is not exhibited when running under Windows Powershell.

.github/workflows/build.yml

@@ -35,6 +35,7 @@ jobs:
           key: ${{ runner.os }}-tools-${{ hashFiles('recipe.cake') }}
 
       - name: Build project
+        shell: powershell
         run: |
           ./build.ps1 --target=CI