Fix DSL#env override covariance error

[MikeMcQuaid]

• typed: strict on build_environment.rb tightened DSL#env to returns(BuildEnvironment) but the Requirement override legitimately returns nil
• Mark DSL#env as overridable with nilable return
• Replace T.must with explicit ArgumentError guard
• Add missing override. to Requirement.env sig

Fixes #22022

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Worked with Claude Code with manual review and testing.

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[Copilot]

Pull request overview

Resolves a Sorbet covariance/typechecking error around BuildEnvironment::DSL#env so that Requirement.env can legitimately return nil when used in its block form.

Changes:

• Make BuildEnvironment::DSL#env overridable and widen its return type to T.nilable(BuildEnvironment) to support nil-returning overrides.
• Replace T.must(@env) with an explicit ArgumentError guard when @env is not initialized.
• Mark Requirement.env’s signature as an explicit override.

Reviewed changes

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

File	Description
Library/Homebrew/build_environment.rb	Widen DSL#env return type for override compatibility and add explicit guard when @env is unset.
Library/Homebrew/requirement.rb	Mark env sig as override to match the intended Sorbet override relationship.

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