Consider adding option to disable cache key fallback

[kyri-petrou]

The current behaviour of the cache is to fallback to a different cache key on cache miss. It would be good if this behaviour could be disabled, or if the user could define a cache key prefix which applies to all cache keys.

The main driver behind this is avoiding unnecessary downloads / uploads for repos that have multiple builds, with some having only a handful of dependencies or different scala versions

coursier cache keys:
  coursier-lint-scala-code-475cd933d4d254d8267377454164c8ad6db554de
  coursier-lint-scala-code-
  coursier-

< Cache download logs >

Cache restored successfully
coursier cache miss, fell back on coursier-build-b930455d2f9b4f12b182a5c0f86b8680b4479aed.
coursier cache will be saved in post run job.

I'm happy to contribute if you agree with adding this feature but don't have the capacity to work on it at the moment - might need a nudge in the right direction though