util: Check whether discovered powershell is actually executable (#43044)

Closes https://github.com/zed-industries/zed/issues/42944

The powershell we discovered might be in a directory with higher
permission requirements which will cause us to fail using it.

Release Notes:

- Fixed powershell discovery disregarding admin requirements
5ccbe945a6Lukas Wirth committed on 11/19/2025, 9:26:49 AM· committed by GitHubparenta910c59
3 files changedLine totals unavailable