fix(ci): replace unpinned third‑party cache action with actions/cache@v4

[micahkendall]

Motivation

• A mutable third‑party action Swatinem/rust-cache@v2 was introduced in Docker-related workflows and created a CI supply‑chain risk because the tag is not pinned to a commit SHA and the publish workflow has elevated/persistent credentials.
• Restore a safe, previously used caching method while preserving per‑target cache behavior to remove the exposure without altering build logic.

Description

• Replaced uses: Swatinem/rust-cache@v2 with uses: actions/cache@v4 in .github/workflows/docker.yml and .github/workflows/refresh-docker-cache.yml.
• Restored the path, key, and restore-keys configuration so caching remains scoped per Rust target and keeps cargo-registry-cache and sccache-cache directories.
• Renamed the step back to Cargo Cache to match prior semantics and kept the compile/build steps unchanged.

Testing

• Ran git diff --check to ensure there are no whitespace/patch-format problems and it succeeded.
• Committed the change with git commit and inspected the commit stat using git show --stat, which succeeded.
• Attempted to parse the workflow YAML with python + yaml.safe_load but it failed due to the environment missing the PyYAML module (ModuleNotFoundError), so automated YAML schema validation was not executed.

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Codex Task