fix rust-cache with proper key and global RUSTFLAG

[swanandx]

CI isn't caching properly. let's say a PR is opened, job runs and saves cache with key "-c4efedb3", now if I push a commit ( editing changelog ), it has restore key of "-fda4c98d.." so it never finds the cache which it should have!

Basically, we are never seeing an cache hit.

This solves two things:

• use keys with runner.os ( for cache hits )
• set `RUSTFLAG: -D warnings" globally instead of just clippy ( otherwise the cache is invalidated and project is rebuilt every time! )

Type of change

• Miscellaneous (related to maintenance)

Checklist:

• Formatted with cargo fmt
• Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.