sg: wait for cmd readers to close before waiting on cmd

Created by: mrnugget

The docs say that calling cmd.Wait() before waiting for the readers to close is incorrect:

Wait will close the pipe after seeing the command exit, so most callers need not close the pipe themselves. It is thus incorrect to call Wait before all reads from the pipe have completed. For the same reason, it is incorrect to call Run when using StdoutPipe. See the example for idiomatic usage.

See: https://pkg.go.dev/os/exec#example-Cmd.StdoutPipe