std.process.Child: fix spawning child proc with new cwd fd

Before this fix, the dup2 of the progress pipe was clobbering the cwd
fd, causing the fchdir to return ENOTDIR in between fork() and exec().
947a3a1be9Andrew Kelley committed on 5/28/2024, 3:56:49 AMparentb7889f2
1 file changedLine totals unavailable