PowerShell hands child processes a console with
ENABLE_VIRTUAL_TERMINAL_PROCESSING off, so conhost drew the progress
line's ESC[2K as a glyph: every install.ps1 status line printed as
'<-[2K✓ ffmpeg'. The progress stream now opts its console handle in to VT
before use, and a handle that is not a console (NUL reports isatty()
too) gets plain lines instead.
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