CI: Use "bash" when running on Windows

Author: amotl

.github/workflows/main.yml

@@ -32,6 +32,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install dependencies
+        shell: bash
         run: |
           python -m pip install --upgrade pip
           python bootstrap.py
@@ -53,6 +54,7 @@ jobs:
           bin/buildout -n -c base.cfg
 
       - name: Test
+        shell: bash
         run: |
           bin/flake8