Powershell scan takes a long time

### Describe the bug.

Please see https://github.com/dotnet/vscode-dotnet-runtime/issues/2171#issuecomment-2992232523

> 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.

### Please attach log file(s) here if you have any.

The log file location should be copied to your clipboard if you came from VS Code.

### Reproduction Steps

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### Expected Behavior

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### Exceptions & Errors (if any)

_No response_

### .NET Install Tool Version

.

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

N/A

### VS Code Version

_No response_