[ci] Generate a different cache key for every job

[afilini]

Without this fix every job was using essentially the same cache key, which meant that they were all using the cache uploaded by the first runner to finish whenever we made some changes to Cargo.toml.

This fix should in theory yield a nice performance boost, especially on jobs that are particularly slow, like when compact_filters is enabled.

If this works out, we can close #157 because after the first run the compiled librocksdb-sys should stay cached for all the subsequent ones.

[RCasatta]

ack 0edcc83