ci: ensure the registry cache and lockfile are up-to-date

We are caching the CARGO_HOME which includes git repos of the gtk-rs bindings. Since we don't specify a branch so it uses the default branch for the ref.

cargo build usually does an update if it has network access, but its not guaranted or the case for other tools like clippy which may still point to the stale git snapshot like here:

https://gitlab.freedesktop.org/thaytan/gstreamer-rs/-/jobs/4165449