system_config: show core cask tap output on Linux

[AlternateRT]

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

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Linux has supported casks for a while now. With these changes, brew config should now output the same core cask tap info shown on macOS.

Sample brew config output

% brew config
HOMEBREW_VERSION: 6.0.15-51-gb36f326
ORIGIN: https://github.com/Homebrew/brew
HEAD: b36f3269ee1c071bba854b547146ad0a89fd5678
Last commit: 15 minutes ago
Branch: config-show-cask-info-on-linux
Core tap JSON: 04 Aug 15:24 UTC
Core cask tap HEAD: 97274269f80b61dd67063780a4a3951d7d75b9d8
Core cask tap last commit: 44 minutes ago
HOMEBREW_PREFIX: /home/linuxbrew/.linuxbrew
Homebrew Ruby: 4.0.6 => /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/4.0.6/bin/ruby
CPU: hexa-core 64-bit arm
Clang: N/A
Git: 2.53.0 => /bin/git
Curl: 8.18.0 => /bin/curl
Kernel: Linux 7.0.0-28-generic aarch64 GNU/Linux
Landlock ABI: 8
OS: Ubuntu 26.04 LTS (resolute)
Host glibc: 2.43
Host libstdc++: 6.0.35
/usr/bin/gcc: 15.2.0
/usr/bin/ruby: N/A
glibc: N/A
gcc@13: N/A
gcc: N/A
xorg: N/A

[MikeMcQuaid]

Thanks!