Build/Test Tools: Double escape quotation marks(“) for Slack “messages”.

This prevents quotation marks from producing invalid JSON errors.

See #52644.

git-svn-id: https://develop.svn.wordpress.org/trunk@51709 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/slack-notifications.yml

@@ -63,7 +63,7 @@ jobs:
       - name: Prepare commit message.
         id: commit-message
         run: |
-          COMMIT_MESSAGE=$(cat <<'EOF' | awk 'NR==1' | sed 's/`/\\`/g' | sed 's/\"/\\"/g'
+          COMMIT_MESSAGE=$(cat <<'EOF' | awk 'NR==1' | sed 's/`/\\`/g' | sed 's/\"/\\\\"/g'
           ${{ github.event.workflow_run.head_commit.message }}
           EOF
           )