Speed up CI

Our CI pipeline runs in 15 minutes, which is crazy for such a small project.

We should work to fix this. Some of this is due to `credentialsd-common` building twice and `credentialsd` because of Meson/Cargo weirdness. But there are some things that we could probably improve.

See https://matklad.github.io/2021/09/04/fast-rust-builds.html for some inspiration.