Enable Cargo's sparse registry for CI

[TimJentzsch]

Closes #260.

This PR enables Cargo's sparse registry which can speed up the dependency updates in some cases.

The also https://blog.rust-lang.org/inside-rust/2023/01/30/cargo-sparse-protocol.html.

Depends on #262 for fixing CI.

[janhohenheim]

Looks like this saves us 3-4 mins on test-native-docs :D

[TimJentzsch]

Looks like this saves us 3-4 mins on test-native-docs :D

That's more than I expected.
Are you sure this change isn't also caused by the cache?
I.e. that we got a cache miss in the previous run?

[janhohenheim]

Could be. In my experience, test-native-docs is a bit weird in general judging by what does and does not factor into its caching.

[TimJentzsch]

I also think parts of the cache are not working correctly for some of our CI workflows...
It's really strange how long CI is taking for us