Add .NET install workaround for RTM

[adityapatwardhan]

PR Summary

Add a conditional step in nuget.yml to install .NET 5 RTM pre-release build.

PR Context

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• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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  • Documentation needed
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  • N/A or can only be tested interactively
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  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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[ghost]

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

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• Release Notes

[ghost]

🎉v7.2.0-preview.1 has been released which incorporates this pull request.:tada:

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• Release Notes