Simplify `dev.nix` #14162

Job	Run time
Test Flutter stable on ubuntu-latest	28m 13s
Test Flutter stable on macos-latest	19m 24s
Test Flutter stable on windows-latest	41m 18s
Test Flutter beta on ubuntu-latest	26m 37s
Test Flutter beta on macos-latest	20m 9s
Test Flutter beta on windows-latest	40m 1s
Test Flutter master on ubuntu-latest	29m 36s
Test Flutter master on macos-latest	27m 24s
Test Flutter master on windows-latest	51m 15s
	4h 43m 57s

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