CI: run Complement on the VM, not inside Docker (#11811)

* CI: run Complement on the VM, not inside Docker

This requires https://github.com/matrix-org/complement/pull/289

We now run Complement on the VM instead of inside a Docker container.
This is to allow Complement to bind to any high-numbered port when it
starts up its own federation servers. We want to do this to allow for
more concurrency when running complement tests. Previously, Complement
only ever bound to `:8448` when running its own federation server. This
prevented multiple federation tests running at the same time as they would
fight each other on the port. This did however allow Complement to run
in Docker, as the host could just port forward `:8448` to allow homeserver
containers to communicate to Complement. Now that we are using random
ports however, we cannot use Docker to run Complement. This ends up
being a good thing because:
 - Running Complement tests locally is closer to how they run in CI.
 - Allows the `CI` env var to be removed in Complement.
 - Slightly speeds up runs as we don't need to pull down the Complement
   image prior to running tests. This assumes GHA caches actions sensibly.

* Changelog

* Full stop

* Update .github/workflows/tests.yml

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

* Review comments

* Update .github/workflows/tests.yml

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>

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