Fix cleanup ruby logic

[maxim-belkin]

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Fixes #7743

Align the logic in cleanup_portable_ruby with that in ruby.sh.

[MikeMcQuaid]

Looking good so far but, stepping back, I wonder if this could all be a Bash function we could shell out to run instead? Shelling out to brew shellenv only takes 0.3s on my system and if we called ruby.sh directly we could perhaps be even faster.

[MikeMcQuaid]

Nice work on this so far, @maxim-belkin!

[maxim-belkin]

Thanks, @MikeMcQuaid. Why did you add .3 to HOMEBREW_REQUIRED_RUBY_VERSION?

[MikeMcQuaid]

@maxim-belkin That's the required system version on macOS (that we're checking in brew doctor) and it's easier to discard the parts we don't need for supported versions.

[MikeMcQuaid]

Let's see how this goes, thanks for your work on this @maxim-belkin!