Add beta channel to CI

[craiglabenz]

Includes beta channel in the CI suite.

CI will never pass on this PR until Flutter's native assets feature graduates out of experiment status. As long as $ flutter config --enable-native-assets is a thing, Flutter's MediaPipe libraries will only work on the master channel, even as that channel progresses.

[domesticmouse]

Beta CI pipeline isn't happy.

❌ loading /Users/runner/work/flutter-mediapipe/flutter-mediapipe/packages/mediapipe-task-text/test/text_classifier_result_test.dart (failed)
  Error: Package(s) mediapipe_text require the native assets feature to be enabled. Enable using `flutter config --enable-native-assets`.
  package:flutter_tools/src/base/common.dart 10:3                                 throwToolExit
  package:flutter_tools/src/isolated/native_assets/native_assets.dart 255:5       nativeBuildRequired
  ===== asynchronous gap ===========================
  package:flutter_tools/src/isolated/native_assets/macos/native_assets.dart 93:8  buildNativeAssetsMacOS
  ===== asynchronous gap ===========================
  package:flutter_tools/src/isolated/native_assets/test/native_assets.dart 40:31  testCompilerBuildNativeAssets
  ===== asynchronous gap ===========================
  package:flutter_tools/src/test/test_compiler.dart 172:37                        TestCompiler._onCompilationRequest
  TimeoutException after 0:12:00.000000: Test timed out after 12 minutes.
  package:test_api/src/backend/invoker.dart 338:28  Invoker._handleError.<fn>
  dart:async/zone.dart 1391:47                      _rootRun
  dart:async/zone.dart 1301:19                      _CustomZone.run
  package:test_api/src/backend/invoker.dart 336:10  Invoker._handleError
  package:test_api/src/backend/invoker.dart 291:9   Invoker.heartbeat.<fn>.<fn>
  dart:async/zone.dart 1399:13                      _rootRun
  dart:async/zone.dart 1301:19                      _CustomZone.run
  package:test_api/src/backend/invoker.dart 290:38  Invoker.heartbeat.<fn>
  dart:async-patch/timer_patch.dart 18:15           Timer._createTimer.<fn>
  dart:isolate-patch/timer_impl.dart 398:19         _Timer._runTimers
  dart:isolate-patch/timer_impl.dart 429:5          _Timer._handleMessage
  dart:isolate-patch/isolate_patch.dart 184:12      _RawReceivePort._handleMessage

[craiglabenz]

@domesticmouse This PR is a placeholder for when native-assets migrates out of experiment status - it cannot be make to work while $ flutter config --enable-native-assets is a thing.

[domesticmouse]

@domesticmouse This PR is a placeholder for when native-assets migrates out of experiment status - it cannot be make to work while $ flutter config --enable-native-assets is a thing.

Suggestion: mark this PR as a draft PR