On NixOS, `bash` isn't under `/bin/bash` but rather in some directory in `$PATH`. Locally, I've been patching those scripts to make them work.

`/usr/bin/env` seems to be the only [portable way](https://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my

) to use binaries from the PATH as interpreters.

Signed-off-by: Quentin Gliech <quentingliech@gmail.com>

* Don't use a virtualenv

* Generate the server's signing key to allow it to start

* Add signing key paths to CI configuration files

* Use a Python script to create the postgresql database

* Improve logging

This adds:

* a test sqlite database
* a configuration file for the sqlite database
* a configuration file for a postgresql database (using the credentials in `.buildkite/docker-compose.pyXX.pgXX.yaml`)

as well as a new script named `.buildkite/scripts/test_synapse_port_db.sh` that:

1. installs Synapse
2. updates the test sqlite database to the latest schema and runs background updates on it
3. creates an empty postgresql database
4. run the `synapse_port_db` script to migrate the test sqlite database to the empty postgresql database (with coverage)

Step `2` is done via a new script located at `scripts-dev/update_database`.

The test sqlite database is extracted from a SyTest run, so that it can be considered as an actual homeserver's database with actual data in it.