Fix cask upgrade env conflict

[MikeMcQuaid]

• Avoid treating the developer default as an explicit HOMEBREW_UPGRADE_AUTO_UPDATES_CASKS opt-in.
• Preserve the existing error when users explicitly set both HOMEBREW_UPGRADE_AUTO_UPDATES_CASKS and HOMEBREW_NO_UPGRADE_AUTO_UPDATES_CASKS outside developer mode.

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[Copilot]

Pull request overview

This PR adjusts cask upgrade environment-variable conflict handling so developer-mode defaults don’t incorrectly trigger a mutual-exclusion error, while keeping the existing error for non-developer users who explicitly set both variables.

Changes:

• In Cask::Upgrade.outdated_casks, only raise on HOMEBREW_UPGRADE_AUTO_UPDATES_CASKS + HOMEBREW_NO_UPGRADE_AUTO_UPDATES_CASKS conflicts when not in developer mode.
• Add an RSpec example covering the developer-mode case where both variables are set (to reflect brew.sh exporting the default).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/cask/upgrade.rb	Skips the env-var conflict error in developer mode to allow opt-out to override the developer default.
Library/Homebrew/test/cask/upgrade_spec.rb	Adds a regression spec for the developer-mode env-var interaction.

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