chore(repo): excludes flutter/dart sdk beta testing on PRs #840

Job	Run time
test / test-dart-vm (3.0.1)	1m 57s
test / test-dart-vm (stable)	2m 17s
ddc_test / test-dart-ddc (3.0.1, chrome)	2m 11s
ddc_test / test-dart-ddc (3.0.1, firefox)	2m 3s
ddc_test / test-dart-ddc (stable, chrome)	3m 40s
ddc_test / test-dart-ddc (stable, firefox)	2m 1s
native_test / test-dart-native (macos-latest)	2m 58s
native_test / test-dart-native (windows-latest)	3m 13s
dart2js_test / test-dart-2js (3.0.1, chrome)	4m 36s
dart2js_test / test-dart-2js (3.0.1, firefox)	2m 37s
dart2js_test / test-dart-2js (stable, chrome)	3m 0s
dart2js_test / test-dart-2js (stable, firefox)	2m 37s
	33m 10s

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