Cache Dependencies for CI

[AlexanderPortland]

As far as I can tell we don't take advantage of GitHub's caching feature for CI. I think even just caching the cargo registry & kani dependencies could help with our build times and, since caches can be shared between workflows, this could save on redundant work happening between them.

Although builds are only a small portion of the time it takes longer running workflows to run (e.g. building is just ~3 min of 32 total in this run of the regression tests), this could make a significant difference on shorter workflows like clippy, the benchcomp tests, and docs building.