Bundle: use full formula name for upgrade checks

[ooye-sanket]

This PR fixes a bug where brew bundle fails to upgrade tapped formulae when they shadow a homebrew-core formula name.

The Problem:
The upgradable? method in formula_installer.rb was using @name (short name like "tart") instead of @full_name (full name like "cirruslabs/cli/tart") when checking if a formula can be upgraded. This caused Formula[formula] to resolve to the deprecated homebrew-core formula instead of the intended tapped formula.

The Fix:
Changed line 237 to use @full_name instead of @name, ensuring Formula[] resolves to the correct tapped formula.

Fixes #21521

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[MikeMcQuaid]

Thanks!