ci: Fix release workflows (#901)

Author: mtrezza

.github/workflows/release-automated.yml

@@ -29,7 +29,7 @@ jobs:
         if: env.package == 'flutter'
         uses: subosito/flutter-action@v2
         with:
-          channel: 'beta'
+          channel: 'stable'
           cache: true
       - name: Install dart dependencies
         run: dart pub get --directory 'packages/dart'

.github/workflows/release-manual.yml

@@ -32,7 +32,7 @@ jobs:
         if: env.package == 'flutter'
         uses: subosito/flutter-action@v2
         with:
-          channel: 'beta'
+          channel: 'stable'
           cache: true
       - name: Install dart dependencies
         run: dart pub get --directory 'packages/dart'