ci: add sccache GHA backend to check-msrv job (experiment)
Layers sccache (mozilla-actions/sccache-action, GHA cache backend) on top
of the Swatinem/rust-cache restored in #14196 for the check-msrv job only.
check-msrv is ~90% cargo check by wall time and runs on fast GitHub-hosted
x64, making it the cleanest signal to test whether sccache adds a further
speedup on top of rust-cache before considering test/check-v2 (where
clippy dominates and sccache can't cache clippy's workspace-crate
invocations).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RZBbswnygEHMcVfzfEDgJc
