Prevent GitHub workflow for daily dotnet build updates from running in forks

[bergmeister]

PR Summary

See my comment in the original PR here: #12748 (comment)
I haven't tested it though and just taken the syntax from the documentation.

cc @adityapatwardhan

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[TravisEz13]

The workflows in powershell/homebrew-tap have this bug..

[ghost]

🎉v7.1.0-preview.4 has been released which incorporates this pull request.:tada:

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