Speed up CI

[iinuwa]

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.

[iinuwa]

Updated to use a Cargo workspace and cut out some unnecessary dependencies here and on libwebauthn. That increased parallelism and decreased number of units to compile. We also set up caching and now use mold as our linker. All of this brought us down to about 6 minutes of CI time, which isn't great but is much better.

We can try splitting the lints into a separate workflow, and/or skipping cargo build since cargo test already builds, but with a separate configuration which causes a rebuild.

[iinuwa]

I moved the build directory out of the source tree in response to #192. It doesn't seem to make a big difference, but pinning the Rust version does seem to help with cache hits. I'll go ahead and close this as completed, since we're down to 4-5 minutes for CI now.