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Adds upstream-repo guards to the scheduled issue jobs in:

• sbom.yml
• sorbet.yml
• spdx.yml
• sponsors-maintainers-man-completions.yml

These workflows already skip their main scheduled jobs on forks, but the companion issue jobs still ran on schedule and treated skipped as actionable, causing failing scheduled runs on forks. (e.g., https://github.com/shaanmajid/brew/actions/runs/22424596092, https://github.com/shaanmajid/brew/actions/runs/22424360798)