test/cmd/update_spec: fix local test run failures

[cho-m]

brew tests is a developer command so running it triggers HOMEBREW_DEV_CMD_RUN and causes failures for some tests set up for stable brew tags.

Mainly to allow running tests locally for development purposes. Otherwise can see failures like following (at least in Linux container):

     1.1) Failure/Error: expect(stderr).to be_empty
            expected `"unexpected curl https://api.github.com/repos/Homebrew/brew/commits/main\n".empty?` to be truthy, got false
          # ./test/cmd/update_spec.rb:316:in 'block (2 levels) in <main>'

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[Copilot]

Pull request overview

This PR makes Library/Homebrew/test/cmd/update_spec.rb resilient to being executed in an environment where HOMEBREW_DEV_CMD_RUN is set (e.g. when running the test suite via the developer command brew tests). It ensures the update script behaves as the specs expect (stable-tag update behavior) when spawned via Open3.capture3.

Changes:

• Explicitly unset HOMEBREW_DEV_CMD_RUN in the environment passed to run_update_shell for the affected examples.
• Prevents unexpected curl requests to .../commits/main during these tests when HOMEBREW_DEV_CMD_RUN would otherwise be inherited.

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