system_command: avoid modifying Context.current

[cho-m]

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May need to know if original code was intentional. But at least seems to fix reported issue for me:

Before

❯ rm -rf "$(brew --cache)/api" && HOMEBREW_DOWNLOAD_CONCURRENCY=auto brew outdated
✔︎ JSON API formula_tap_migrations.jws.json
✔︎ JSON API cask_tap_migrations.jws.json
✔︎ JSON API cask.jws.json
✔︎ JSON API formula.jws.json
ca-certificates
clang-format
gh
libomp
openexr

After

❯ rm -rf "$(brew --cache)/api" && HOMEBREW_DOWNLOAD_CONCURRENCY=auto brew outdated
✔︎ JSON API formula_tap_migrations.jws.json
✔︎ JSON API cask_tap_migrations.jws.json
✔︎ JSON API cask.jws.json
✔︎ JSON API formula.jws.json
ca-certificates (2025-09-09) < 2025-11-04
clang-format (21.1.4) < 21.1.5
gh (2.82.1) < 2.83.0
libomp (21.1.4) < 21.1.5
openexr (3.4.2_1) < 3.4.3

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MikeMcQuaid edit: fixes #20954

[Copilot]

Pull Request Overview

This PR updates the context assignment mechanism in SystemCommand to use thread-local storage instead of the module-level setter when passing context to background threads.

• Changed from Context.current = to Thread.current[:context] = for thread context inheritance

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[MikeMcQuaid]

This makes sense to me, thanks a lot @cho-m! 😍

I suspect it's just that the previous code was usually just single-threaded.

Let's try this out and can revert if it causes more issues.