Specify bash shell

Co-authored-by: Alex Waygood <[email protected]>

Author: hugovk

.github/workflows/reusable-windows-msi.yml

@@ -25,3 +25,4 @@ jobs:
         persist-credentials: false
     - name: Build CPython installer
       run: .\Tools\msi\build.bat --doc -"${ARCH}"
+      shell: bash