Powershell scan takes a long time

[nagilson]

Describe the bug.

Please see #2171 (comment)

when we check if powershell is available, this is taking a very long time, several seconds (7 seconds spent validating powershell with shared thread.) We should simplify that logic to check if the file exists for the powershell exectuable first and only run the more thorough validation if it fails the first time.

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Reproduction Steps

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Expected Behavior

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Exceptions & Errors (if any)

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.NET Install Tool Version

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Your operating system? Please provide more detail, such as the version and or distro above.

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VS Code Version

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