Replace local detect-runner-os action with external detect-versions@v1 to
reduce custom action maintenance. Add architecture detection for future
cross-platform support and namespace all cache keys with "continuwuity-"
prefix to prevent collisions with other projects on shared runners.
Updates cache mount IDs in Dockerfiles to match the new namespacing
convention, ensuring consistent cache isolation across CI and Docker builds.
Also moves rustup installation to a seperate workflow and
enables caching.
The sccache action required a github.com api token, so we set
all that up too.
