edit: pipe commands assume that they can pass the -c flag to the shell.

On Windows, the shell is PowerShell.exe, and its flag is -Command. We can probably create shell_$(GOOS).go files to define DefaultShell and CommandFlag. On Unix DefaultShell="/bin/sh" and CommandFlag="-c". On Windows DefaultShell="PowerShell.exe" and CommandFlag="-Command".

Start by locating the pipe-command code that invokes the shell and inspect how it currently supplies the shell executable and flag. Add platform-specific definitions for DefaultShell and CommandFlag as described for Unix and Windows, then verify that pipe commands use the correct shell flag on both platforms.

Written by the indexing model from the issue text.