GitHub workflow is failing due to lack of macOS Big Sur (macOS 11) support

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GitHub: we don't have plans to add macOS 11 beta (usually, we add new macOS only after stable release due to maintenance concerns). However, could you please create an issue for this request? It will help us to track popularity of this request.

Currently supported macOS version (10.15)

For reference: runs-on syntax

Also: backslash-f/simage#3

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Fixed by e73b810