Skip building of examples and ceno_rt when running cargo clippy or cargo check

[matthiasgoergens]

cargo check and cargo clippy can be fairly fast, because they do not need to actually build the libraries and executables.

However because of the way examples-builder makes our RiscV examples available to the native parts of Ceno, running cargo check or cargo clippy on the native parts leads to a building of the RiscV parts. And that takes a comparatively long time, on a fresh checkout, like in our CI.

This issues tracks our progress on removing this slowdown.