tap-info cmd: skip untapped core taps

[rrotter]

When homebrew/core or homebrew/cask are untapped brew tap-info fails because Tap.each includes them and tap.private? fails without a git repo interrogate.

This restores the behavior of brew tap-info before #16710

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[apainintheneck]

I was able to reproduce this locally.

$ brew untap homebrew/cask
Untapping homebrew/cask...
Untapped 4417 casks (4,965 files, 382.0MB).
$ brew tap-info
Error: No available tap homebrew/cask.
Run brew tap --force homebrew/cask to tap homebrew/cask!

The fix seems to work too.

$ brew tap-info
10 taps, 1 private, 7022 formulae, 16 commands, 17,037 files, 759.5MB

[apainintheneck]

Thanks for your first contribution to Homebrew!