completion: update SHELL for ngrok

[branchv]

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ngrok cask appears to ignore $SHELL unless it looks like a path

$ SHELL=zsh ngrok completion | head -1
# bash completion for ngrok                                -*- shell-script -*-
$ SHELL=/zsh ngrok completion | head -1
#compdef ngrok

I would send ngrok a PR instead, but it's closed source and is simple enough to support on our side

[Copilot]

Pull request overview

This PR adjusts Homebrew’s generated shell completion logic to set SHELL as an absolute path (e.g., /bin/zsh) when invoking completion generators, to accommodate tools (like ngrok) that treat SHELL differently depending on whether it looks like a path.

Changes:

• Set SHELL to /bin/<shell> when generating completions for both formulae and casks.
• Update cask generated-completion specs to reflect the new SHELL value format.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/formula.rb	Changes the SHELL env passed to completion generators to an absolute /bin/<shell> path.
Library/Homebrew/cask/artifact/generated_completion.rb	Applies the same SHELL env change for cask-generated completions.
Library/Homebrew/test/cask/artifact/generated_completion_spec.rb	Updates expectations/mocking to match the new SHELL env format.

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[MikeMcQuaid]

This doesn't seem like the right fix. /bin/bash and /bin/zsh exist on macOS but may not on Linux and /bin/fish doesn't.

[MikeMcQuaid]

This doesn't seem right?

[MikeMcQuaid]

This path doesn't exist

[branchv]

Answered below, but we could set them to paths in homebrew's prefix if you think that makes more sense

[branchv]

/bin/bash and /bin/zsh exist on macOS but may not on Linux and /bin/fish doesn't.

Right, but this path doesn't need to exist here as it's only purpose is for the completion generator to infer what shell to generate for. Just as setting SHELL=fish when fish is not guaranteed to be installed. Does that make sense?

[MikeMcQuaid]

@branchv I don't think we should make a global completion change like this just for an ngrok cask. It seems likely to break more than it will fix, sorry.