[CI] Github Actoins release build caching

[elpiel]

With the introduction of GH Actions in #350, we still have some performance that could be gained when building the Release.
Since the jobs don't share the files nor the tools we currently checkout the code a second time and when cargo build is invoked the cargo updates the registry.

This potentially could be improved by sharing artefacts or using a cargo caching action.

Resources:

• https://github.com/actions/download-artifact
• https://github.com/actions/upload-artifact
• https://github.com/Swatinem/rust-cache (currently always uses the job in the cache key, but do check out Do not append process.env.GITHUB_JOB to the cache key if key is set Swatinem/rust-cache#6 )