ci: map macOS to darwin in bun download URL

.github/actions/setup-bun/action.yml

@@ -16,7 +16,11 @@ runs:
       shell: bash
       run: |
         V=$(node -p "require('./package.json').packageManager.split('@')[1]")
-        OS=$(echo "$RUNNER_OS" | tr '[:upper:]' '[:lower:]')
+        case "$RUNNER_OS" in
+          macOS)   OS=darwin ;;
+          Linux)   OS=linux ;;
+          Windows) OS=windows ;;
+        esac
         echo "url=https://bun.sh/download/${V}/${OS}/x64?avx2=false&profile=false" >> "$GITHUB_OUTPUT"
 
     - name: Setup Bun