This fix is already in master branch for stdin, stdout, and stderr; this
commit solves the same problem but for the progress pipe.
Both fixes were originally included in one commit on this branch,
however it was split it into two so that master branch could receive the
fix before the progress branch is merged.
The added comment explains the issue here relatively well. The new
progress API made this bug obvious because it became visibly clear that
certain Compile steps were seemingly "hanging" until other steps
completed. As it turned out, these child processes had raced to spawn,
and hence one had inherited the other's stdio pipes, meaning the `poll`
call in `std.Build.Step.evalZigProcess` was not identifying the child
stdout as closed until an unrelated process terminated.
