Replace mozilla-actions/sccache-action with a custom Forgejo-specific
implementation that eliminates GitHub token dependencies and rate limiting
issues for all contributors regardless of repository permissions.
The new action mirrors sccache binaries to the Forgejo package registry
and queries that instead of GitHub releases, maintaining identical functionality
including hostedtoolcache support.
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.
