ghostty: don't install package on darwin platforms

[HeitorAugustoLN]

Description

Closes #6295

Makes ghostty package option nullable on darwin platform

Checklist

• Change is backwards compatible.

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Maintainer CC

[teto]

rather than making it nullable, it could be added to home.packages only for supported platforms ?

[HeitorAugustoLN]

Closing in favor of: #6300