`intro_flutter_gpu`: Add Linux runner #14254

Job	Run time
Test Flutter stable on ubuntu-latest	28m 12s
Test Flutter stable on macos-latest	21m 16s
Test Flutter stable on windows-latest	42m 13s
Test Flutter beta on ubuntu-latest	28m 58s
Test Flutter beta on macos-latest	20m 34s
Test Flutter beta on windows-latest	42m 51s
Test Flutter master on ubuntu-latest	29m 29s
Test Flutter master on macos-latest	24m 37s
Test Flutter master on windows-latest	46m 45s
	4h 44m 55s

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