Merge separate lint and test jobs into a single check job so clippy's
compilation artifacts are reused by tests. Add actions/cache for the
Cargo target directory and registry to enable incremental builds across
runs. Drop unnecessary fetch-depth: 0 from CI jobs.
The container image is tagged with the package version from flake.nix,
not the git short hash. Read the version from Cargo.toml so the
docker tag command references the correct source image.
