[CI] Cache ~/.cargo dir between CI runs

[SpotlightKid]

Compiling rust is slow and rust packages often have literally hundreds of dependency packages, which must be downloaded and compiled on each CI run.

We can at least speed up the downloading part by caching the ~/.cargo directory between CI runs.

Things to consider:

• Does this affect CI steps, which run concurrently, like "Build packages" and "Reproduce packages"
• Do we need to make specials provisions for different architectures?

[cbix]

I recently configured the CI runners to mount a persisted shared cache directory in all CI steps under /drone/cache (and set $DRONE_CACHE) so we could facilitate that for rust by somehow telling it to use ${DRONE_CACHE:-/var/cache}/cargo as a cache. #598 sets this up for pacman, together with some cleanup.