api: fix every formula having a post install defined

[Bo98]

We compact_blank the hash so false is omitted and never overrides the default of true.

I'm surprised nobody noticed the slowdown.

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[Copilot]

Pull request overview

Adjusts Homebrew::API::FormulaStruct so post_install_defined correctly defaults when the key is omitted from API hashes due to compact_blank/deep_compact_blank, avoiding treating every formula as having post_install defined (and the associated slowdown).

Changes:

• Change post_install_defined default from true to false in Homebrew::API::FormulaStruct.

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