Prevent `start_for_complement.sh` from setting `START_POSTGRES` to `false` when it's already set (#16985)

I have a use case where I'd like the Synapse image to start up a
postgres instance that I can use, but don't want to force Synapse to use
postgres as well.

This commit prevents postgres from being started when it has already
been explicitly enabled elsewhere.

When building the docker images for complement testing, copy a preinstalled
complement over from a base image, rather than apt installing it. This avoids
network traffic and is much faster.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>