Use a single sed invocation instead of repeated in setup_etc_shadow.

- No longer use sudo_multi; instead build up a single sed command and
  run it with sudo directly.

BUG=b:290992228,b:297037274
TEST=Run all presubmits.

Change-Id: I4d30678a803cc33601f9f339aaa1cf1ad4c4b9a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4828614
Tested-by: Scott Haiden <scotthaiden@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Scott Haiden <scotthaiden@google.com>