fix eval (brew shellenv) from exiting with status 1 in fish

[J-M0]

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fish's set doesn't modify the exit status when assigning a variable. This means that set -q VAR; or set VAR ... will return an exit status of 1 even if the variable is suceessfully set. By switching the commands to ! set -q VAR; and set VAR ..., this prevents the 1 from propogating when running eval (brew shellenv).

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @J-M0!

[buildpulse]

Known disruptive tests have failed for this pull request.

Commit SHA: affc4c0
Build URL: https://github.com/Homebrew/brew/actions/runs/6372388989/attempts/1

Suite Name	Test Name	Details
rspec	SystemCommand with both STDOUT and STDERR output from upstream without print_stderr its result stderr is expected to eq "2\n4\n6\n"	→

[buildpulse]

Known disruptive tests have failed for this pull request.

Commit SHA: affc4c0
Build URL: https://github.com/Homebrew/brew/actions/runs/6372388989/attempts/1

Suite Name	Test Name	Details
rspec	SystemCommand with both STDOUT and STDERR output from upstream with default options echoes only STDERR	→