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().

Author: andrewrk

lib/std/process/Child.zig

@@ -654,14 +654,17 @@ fn spawnPosix(self: *ChildProcess) SpawnError!void {
         setUpChildIo(self.stdin_behavior, stdin_pipe[0], posix.STDIN_FILENO, dev_null_fd) catch |err| forkChildErrReport(err_pipe[1], err);
         setUpChildIo(self.stdout_behavior, stdout_pipe[1], posix.STDOUT_FILENO, dev_null_fd) catch |err| forkChildErrReport(err_pipe[1], err);
         setUpChildIo(self.stderr_behavior, stderr_pipe[1], posix.STDERR_FILENO, dev_null_fd) catch |err| forkChildErrReport(err_pipe[1], err);
-        if (prog_pipe[1] != -1) posix.dup2(prog_pipe[1], prog_fileno) catch |err| forkChildErrReport(err_pipe[1], err);
 
         if (self.cwd_dir) |cwd| {
             posix.fchdir(cwd.fd) catch |err| forkChildErrReport(err_pipe[1], err);
         } else if (self.cwd) |cwd| {
             posix.chdir(cwd) catch |err| forkChildErrReport(err_pipe[1], err);
         }
 
+        // Must happen after fchdir above, the cwd file descriptor might be
+        // equal to prog_fileno and be clobbered by this dup2 call.
+        if (prog_pipe[1] != -1) posix.dup2(prog_pipe[1], prog_fileno) catch |err| forkChildErrReport(err_pipe[1], err);
+
         if (self.gid) |gid| {
             posix.setregid(gid, gid) catch |err| forkChildErrReport(err_pipe[1], err);
         }