tap: handle TapCoreRemoteMismatchError when tapping the core tap without a URL

[XuehaiPan]

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Handle TapCoreRemoteMismatchError on brew tap homebrew/core without a URL while HOMEBREW_CORE_GIT_REMOTE set.

$ rm -rf "$(brew --repo homebrew/core)"

$ export HOMEBREW_CORE_GIT_REMOTE="https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/linuxbrew-core.git"

$ brew tap homebrew/core  # tap without a URL while `HOMEBREW_CORE_GIT_REMOTE` set
Updating Homebrew...
Error: Tap homebrew/core remote does mot match HOMEBREW_CORE_GIT_REMOTE.
https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/linuxbrew-core.git != https://github.com/Homebrew/linuxbrew-core
Please set HOMEBREW_CORE_GIT_REMOTE="https://github.com/Homebrew/linuxbrew-core" and run `brew update` instead.
Error: Failure while executing; `/home/PanXuehai/Projects/brew/bin/brew tap homebrew/core` exited with 1.
Error: Tap homebrew/core remote does mot match HOMEBREW_CORE_GIT_REMOTE.
https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/linuxbrew-core.git != https://github.com/Homebrew/linuxbrew-core
Please set HOMEBREW_CORE_GIT_REMOTE="https://github.com/Homebrew/linuxbrew-core" and run `brew update` instead.