runtime: don't race cmd.Wait and cmd.StdoutPipe read

runtime: don't race cmd.Wait and cmd.StdoutPipe read

os/exec.Cmd.Wait closes the read end of os/exec.Cmd.StdoutPipe, meaning
that io.ReadAll can return fs.ErrClosed if the child exits too early,
allowing Wait to complete. The StdoutPipe docs already note this sharp
edge.

Move cmd.Wait until after we finish waiting on stdout. If the child
crashes for some reason, the write end of the pipe will implicitly close
causing io.ReadAll to return as well, so we won't get stuck.

Fixes #52725.

Change-Id: Ifae5745d86206879af2f1523a664236972e07252
Reviewed-on: https://go-review.googlesource.com/c/go/+/420597
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>