Fix linuxbrew-core migration not applying to Linuxbrew/homebrew-core

[Bo98]

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There's an even older remote we used, Linuxbrew/homebrew-core, which redirects to Homebrew/linuxbrew-core. Some users who installed a long time ago may still have Linuxbrew/homebrew-core as a git origin as we never migrated them to Homebrew/linuxbrew-core and simply relied on GitHub's redirect feature. Those users simply faced their linuxbrew-core install no longer receiving updates when we should have been migrating them.

See Homebrew/discussions#2624.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, nice work @Bo98!