Use bash as default shell

The bash shell comes with -o pipefail applied by default.¹

¹ <https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference>

.github/workflows/ci.yaml

@@ -37,6 +37,9 @@ jobs:
 
     name: build and test (os=${{ matrix.os }})
     runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: bash
     steps:
       - name: Install Micromamba
         run: |