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Hi,

after #21278 was merged, I tested the main branch inside our company.

I can't tell what different since it published my initial PR, but now I got an error if I set HOMEBREW_DOCKER_REGISTRY_BASIC_AUTH_TOKEN to none:

/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/global.rb:38:in 'fetch': key not found: "HOMEBREW_GITHUB_PACKAGES_AUTH" (KeyError)
        from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/global.rb:38:in '<top (required)>'
        from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:18:in 'Kernel#require_relative'
        from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:18:in '<main>'

After configure a default value in ENV.fetch, everything works as expected. (tested internally).