[ci] setup flutter in batch release workflow

.github/workflows/batch_release_pr.yml

@@ -10,6 +10,8 @@ jobs:
     env:
       BRANCH_NAME: ${{ github.event.client_payload.package }}-${{ github.run_id }}-${{ github.run_attempt }}
     steps:
+      - name: "Install Flutter"
+        uses: ./.github/workflows/internals/install_flutter
       - name: checkout repository
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8
       - name: Set up tools

.github/workflows/internals/install_flutter/action.yml

@@ -0,0 +1,20 @@
+name: "Install Flutter"
+description: "Installs a pinned version of Flutter"
+runs:
+  using: "composite"
+  steps:
+    - name: "Install Flutter"
+      # This uses a pinned version of Flutter rather than `stable` so that it is
+      # not subject to out-of-band failures when new releases happen. It does
+      # not use the auto-rolled pin because there's no way for the autoroller
+      # to test the actual release flow, so changes would still show up in
+      # post-submit. A manually-rolled pin means that any changes here must be
+      # made deliberately, so that the person updating it knows to watch the
+      # next actual auto-release to ensure that it works, and knows to revert
+      # the change if it doesn't.
+      shell: bash
+      run: |
+        cd $HOME
+        git clone https://github.com/flutter/flutter.git --depth 1 -b 3.38.5 _flutter
+        echo "$HOME/_flutter/bin" >> $GITHUB_PATH
+        cd $GITHUB_WORKSPACE