ptyfwd: Always prioritize $SYSTEMD_TINT_BACKGROUND

ptyfwd: Always prioritize $SYSTEMD_TINT_BACKGROUND

By giving priority to --background= we prevent users from opting
out of coloring if an explicit color is chosen by a tool wrapping
one of our own tools. Instead, let's give priority to the environment
variable, so that even if our tools are wrapped by another tool with
a different background, users can still opt out of coloring just by
setting the environment variable, which has a high chance of being
forwarded to the invocation of our own tools which makes it easy to
use to disable color tinting globally if requested by the user.