Cache dependencies for CI jobs

[AlexanderPortland]

Adds a simple cache of the ~/.cargo/registry directory and Kani's target directory to reduce compilation times of our dependencies in CI. This also caches the ~/.rustup directory to avoid having to re-download toolchains and re-compile the standard library every CI run. Caches are shared between workflows that compile Kani in the same way (e.g. between the benchcomp-tests & perf jobs since they both compile in release mode).

Based on testing on my local fork, this change seems to make a minor, but not insignificant impact on runtimes. Although lost in the noise on longer jobs, llbc-regression & benchcomp-tests got noticeably faster, as did the compiler benchmarking workflows which have to compile Kani for each commit they are comparing.

Test runs:
General Kani CI workflow: before | after
Compiler bench workflow: before | after

Resolves #4153

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tautschnig]

I think that's a good start, although we might want to figure out what else we could save time on (by caching).