Force the use of bash even on Windows for GitHub Actions

Powershell would not support escaping newlines,
and would interpret dots as some object-oriented field accessor.
None of this nonsense, let's just use bash.

.github/workflows/build.yml

@@ -36,8 +36,13 @@ concurrency:
   group: "workflow = ${{ github.workflow }}, ref = ${{ github.event.ref }}, pr = ${{ github.event.pull_request.id }}"
   cancel-in-progress: ${{ github.event_name == 'pull_request' || github.repository != 'hibernate/hibernate-search' }}
 
+defaults:
+  run:
+    shell: bash
+
 env:
   MAVEN_ARGS: "-e -B --settings .github/mvn-settings.xml --fail-at-end"
+
 jobs:
   build:
     name: ${{matrix.os.name}}