Add fix for Start-Job initialization script in system lockdown

[PaulHigin]

PR Summary

This is a Windows platform issue only.

When running remote initialization script, the script block is considered internal and marked as trusted. In system lock down mode, PowerShell runs in ConstrainedLanguage mode and won't allow a trusted script block to run in the default scope.

Fix is to mark the initialization script as untrusted external when system is locked down.

Repro:

# PowerShell running in locked down system
Start-Job -InitializationScript { function Hello { "Hello" } } -ScriptBlock { Hello } | Wait-Job | Receive-Job

# Result
Error: "Cannot dot-source this command because it was defined in a different language mode. To invoke this command without importing its contents, omit the '.' operator."

Fix is to mark initialization script as untrusted "external origin" when system is in lock down mode, so that it can run in ConstrainedLanguage session scope.

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• User-facing changes
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  • User-facing Documentation needed
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• Testing - New and feature
  • Not Applicable or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
    • Add [feature] if the change is significant or affects feature tests

[adityapatwardhan]

@PaulHigin, could you push a commit with [Feature] tag in it. The new test that is added is tagged as Feature hence was not executed in the CI.

[adityapatwardhan]

Restarted CI in appveyor to pickup the latest test fixes.

[PaulHigin]

@adityapatwardhan Can this be merged?