Fix nuget version to 5.8.x

[NlightNFotis]

Previously, we had an issue where the latest default version of
nuget in github actions was broken and couldn't install dependencies.
This was fixed (temporarily) by #6054 when the latest version
installed by setup-nuget was 5.8.1. This has now changed to also
install the bugged version (5.9.x) by default, giving us problems.

These are now solved by temporarily fixing the version of nuget
to a known working version (according to multiple reports online,
and evidence of our own CI logs).

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• My commit message includes data points confirming performance improvements (if claimed).
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