Improve Caching in the GitHub workflow

[utkarshg6]

It turns out that while caching the cargo files in the workflow, we are using the key: ${{ runner.os }}-cargo to uniquely identify and restore the cache.

This has cache invalidation issues, as it doesn't take into account changes in dependencies. To solve it we can introduce the hash of Cargo.lock to uniquely identify the cache based on the lockfile.

Here's a better alternative: key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}