Fix Ubuntu build and runspace test

[bergmeister]

PR Summary

Bringing back my old code to bootstrap the .Net SDK in CI because it works. To me that -bootstrap is over-engineered and therefore hard to debug and does not even work well (it does not persist after closing the shell on Windows, therefore I do not use it for development anyway...)
As part of it it discovered a new test assertion failure (wrong expectation) that it fixes as well.

PR Checklist

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• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
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