This switches the playbook from devture/rust-synapse-compress-state (a
container image which wraps the upstream-prebuilt amd64 binary of
rust-synapse-compress-state) to registry.gitlab.com/mb-saces/rust-synapse-compress-state
(https://gitlab.com/mb-saces/rust-synapse-compress-state), which builds
rust-synapse-compress-state from source and provides a multi-arch image
that currently works on amd64 and arm64.

Ideally, we'll stop using `:latest` and arm32 support will be made
available upstream as well at some point.
Discussed here: https://gitlab.com/mb-saces/rust-synapse-compress-state/-/issues/1