Add sccache and use nextest in our workflow

[ggonzalez94]

Fixes #259

Our workflows take about 50min each on every PR(on main it is longer given we also run it on mac OS). This can be improved using sccache which avoids compilation when possible. It is common in polkadot projects to have this given long build times. With this, subsequent runs take around 35min.

• Add sccache step
• Use nextest for tests, which is up to 3x faster than cargo test, and also shows the results in a much nicer format. It also shows you if you have some slow tests. Currently this is not a big advantage, since our tests are quick, but even the formatting is worth it I think.

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