ci: fix deprecated argument

Author: hpoul

.github/workflows/dart.yml

@@ -16,7 +16,7 @@ jobs:
     - uses: actions/checkout@v1
     - uses: dart-lang/setup-dart@v1
       with:
-            release-channel: ${{ matrix.dart_channel }}
+        sdk: ${{ matrix.dart_channel }}
     - run: |
         which dart
         codesign --remove-signature $(which dart)