chore(cli): Use Dart beta (#357)

Update tests to use Dart beta

Author: dnys1

.github/workflows/celest_cli.yaml

@@ -40,7 +40,8 @@ jobs:
           cache: true
           # Because many golden tests encode the precise Dart/Flutter SDKs used to generate
           # them, these values must be consistently used when running tests locally and in CI.
-          flutter-version: 3.29.3
+          flutter-version: 3.32.0-0.1.pre
+          channel: beta
       - name: Setup Libsecret
         if: runner.os == 'Linux'
         run: tool/setup-ci.sh