corp-ssh-helper-helper: Use shorter socket names

Socket names on linux have a very short maximum path (108 characters).
Calling this from a nested chromiumos directory gives:

  OSError("AF_UNIX path too long")

This commit changes the behaviour to chdir to the chromeos_path
to ensure the socket_path is a consistently short length.

Also update the README.md accordingly.

BUG=None
TEST=I can actually run the server now! Ran tast test in chroot.

Change-Id: Ia0fc3c27ee2954915bf0da07d9ef89016bd8438d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/7913530
Tested-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>