Prepare for opt-in HOMEBREW_SBOM

[MikeMcQuaid]

This functionality is not widely used and it's relatively slow. Let's make it an opt-in for those who need it in 5.2.0.

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OpenAI Codex with several rounds of local tweaking and review.

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[Copilot]

Pull request overview

Prepares Homebrew for making SBOM generation opt-in via a new HOMEBREW_SBOM environment variable, with early wiring for CI/test-bot and TODO markers for the upcoming behavior change in 5.2.0.

Changes:

• Set HOMEBREW_SBOM=1 in brew test-bot’s environment.
• Add HOMEBREW_SBOM to Homebrew::EnvConfig::ENVS (documented as a no-op until 5.2.0).
• Add 5.2.0 TODO/odeprecated notes around SBOM requiring/lazy-loading in FormulaInstaller.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
Library/Homebrew/test_bot.rb	Forces HOMEBREW_SBOM in the test-bot environment.
Library/Homebrew/formula_installer.rb	Adds TODO notes indicating future gating/lazy-loading for SBOM.
Library/Homebrew/env_config.rb	Documents a new HOMEBREW_SBOM boolean env var in EnvConfig.

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