CI: use new dart instead of google/dart image.

Author: greenrobot-team

.github/workflows/dart.yml

@@ -16,7 +16,7 @@ jobs:
   generator:
     runs-on: ubuntu-20.04
     container:
-      image: google/dart:2.17.0
+      image: dart:2.17.0
     steps:
       - uses: actions/checkout@v2
       - name: Install ObjectBox C-API

.github/workflows/lint.yml

@@ -12,7 +12,7 @@ jobs:
   analyze:
     runs-on: ubuntu-20.04
     container:
-      image: google/dart:2.17.0
+      image: dart:2.17.0
     steps:
       - uses: actions/checkout@v2
       - run: dart run build_runner build