Make pip install faster in Docker build for [Complement](https://github.com/matrix-org/complement) testing.

If files have changed in a `COPY` command, Docker will invalidate all of the layers below. So I changed the order of operations to install all dependencies before we `COPY synapse /synapse/synapse/`. This allows Docker to use our cached layer of dependencies even when we change the source of Synapse and speed up builds dramatically! `53.5s` -> `3.7s` builds 

As an alternative, I did try using BuildKit caches but this still took 30 seconds overall on that step. 15 seconds to gather the dependencies from the cache and another 15 seconds to `Installing collected packages`.

Fix https://github.com/matrix-org/synapse/issues/9364