Add required shell property for composite action run commands

Author: aduth

.github/actions/build-design-system-docs/action.yml

@@ -13,7 +13,9 @@ runs:
 
     # 2. Install dependencies for just the design-system-docs package
     - run: yarn workspaces focus --production @automattic/design-system-docs
+      shell: bash
 
     # 3. Build the design-system-docs workspace
     - name: Build design system docs
       run: yarn workspace @automattic/design-system-docs build
+      shell: bash