Use encodedCommand in Invoke-InNewProcress for P-tests

[fflaten]

PR Summary

Invoke-InNewProcress helper-method in P-tests is broken in PowerShell 7.3 because 7.3 now keeps the backslash previously used to escape quotes in the provided scriptblock.

Replacing with encodedCommand (base64) to avoid version-specific logic.

Fixes broken CI mentioned in #2264 (comment) . Some runs use prerelease CI-images with PowerShell 7.3 which makes CI inconsistent atm.

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[fflaten]

Likely caused by this https://learn.microsoft.com/en-us/powershell/scripting/learn/experimental-features?view=powershell-7.3#psnativecommandargumentpassing

@nohwnd Any issues with using powershell -encodedCommand? Might be flagged by some security teams etc, but this only affects our P-tests so hopefully okay. Let me know if I should switch to a workaround in the link above.

[fflaten]

Ubuntu 20.04 and Windows 2019 tests ran with the new image with 7.3 = all good.

[fflaten]

Looks like MS/Github pinned PS 7.2 LTS in newer images atm due to issue like we had, but we should still do something to support future images and local builds using PS7.3+