[flutter] Configure pub.dev OIDC in release job

Author: badlogic

.github/workflows/release-spine-flutter.yml

@@ -22,6 +22,9 @@ jobs:
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
+      - name: Set up Dart for pub.dev OIDC
+        uses: dart-lang/setup-dart@e51d8e571e22473a2ddebf0ef8a2123f0ab2c02c # v1
+
       - name: Set up Flutter
         uses: subosito/flutter-action@v2
         with: