Update UseNewEnvironment parameter behavior of Start-Process cmdlet on Windows

[iSazonov]

PR Summary

The PR effects only Windows behavior.

• With UseNewEnvironment parameter Start-Process run a child process with environment variables without modifications in current session (state before the session start).
  See Start-Process -UseNewEnvironment provides an environment that is missing crucial standard environment variables while not supporting passing a new environment #4671 and PowerShell Committee conclusion UseNewEnvironment restores environment variables for child process #10745 (comment)
• Without UseNewEnvironment parameter Start-Process run a child process with current environment variables from current session. (This allow us easily to add -Environment parameter late)

PR Context

Replace #10561
Fix #3545
Fix partially #4671

PowerShell Committee conclusion #10745 (comment)

PR Checklist

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• Breaking changes
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  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: New UseNewEnvironment parameter behavior of Start-Process cmdlet on Windows MicrosoftDocs/PowerShell-Docs#5774
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  • N/A or can only be tested interactively
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
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[SteveL-MSFT]

Use of CreateEnvironmentBlock() with the user token is great. Did you verify that USERNAME is correct and PATH is complete?

[iSazonov]

We could check USERNAME. Checking PATH it would be that we don't trust native API.

[iSazonov]

Done.

[iSazonov]

@TravisEz13 @anmenaga @SteveL-MSFT Could you please review?

Not applicable

[adityapatwardhan]

@iSazonov would it be possible to validate these changes on nano server?

[iSazonov]

would it be possible to validate these changes on nano server?

Oh, I never have nano. Sorry.

[adityapatwardhan]

Verified it works on docker image for nanoserver.

[ghost]

🎉v7.1.0-preview.2 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes