CI: improve cache re-use

[sveitser]

We should be able to set the shared-key to re-use the cache between jobs. Since we only have a total of 10GB per repo before things get evicted this should be beneficial. Each rust cache takes about 1GB of space.

If we have builds with different cargo profiles we need to make sure they use different caches though. Otherwise we will save a cache with one profile that will then used by another profile and won't contain the necessary files to make the cache effective.