pkg/machine: Read stderr from ssh-keygen correctly

[ashley-cui]

Read stderr from ssh-keygen before calling wait(), since cmd.Wait() closes cmd.StderrPipe() after it exits.

Found this when hunting for the key generation flake from the uncategorized flakes issue. I'm not sure if this fixes it, since no machine flakes have been triggering for me since last Thursday, but since it's a bug it's worth fixing anyway.

Does this PR introduce a user-facing change?

None

[rhatdan]

Would something like:

cmd := exec.Command("...")
  stdout := new(bytes.Buffer)
  stderr := new(bytes.Buffer)

  cmd.Stdout = stdout
  cmd.Stderr = stderr

  if err := cmd.Start(); err != nil {
      ...
  }

  if err := cmd.Wait(); err != nil {
      ...
  }

  in := bufio.NewScanner(io.MultiReader(stdout, stderr))
  for in.Scan() {
      fmt.Println(in.Text())
  }

Make more sense?

[baude]

LGTM

[Luap99]

/lgtm

[openshift-ci]

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