Include an explicit dependency generation in the CI cache key.

This can be incremented whenevery a significant change to the dependencies happens which should not share a CI cache with the existing code to avoid overly large caches.

Doing it this way should avoid both the need to manually clear the caches and for everybody to rebase their branches to include at least the commit doing the dependency bump itself. (If they do not rebase, their dependency generation stays the same, if they do rebase, their dependencies actually match the cache their going to get.)

Finally, this is basically the same mechanism GitLab uses to implement its "Clear CI caches" action but by moving it into the code proper it can be versioned with the dependency specification itself to ensure synchronicity.