Support customizing keys of default caches

Hi!

Sometimes you want to drop the cache without necessarily changing the paths it depends on, so just a "drop cache" button would be nice to have. Sadly, actions/cache doesn't have that ability (actions/cache#2), so we need a workaround.

I used this to work around, but maybe it's something that could be a key? (e.g. the cacheOffset value):

//Increment this every time you want to drop the caches
val cacheOffset = 1

ThisBuild / githubWorkflowGeneratedCacheSteps ~= { steps =>
  steps.map {
    case u @ WorkflowStep.Use("actions", "cache", _, params, _, _, _, _) =>
      u.copy(params = params.map {
        case (k @ "key", cacheKey) => (k, s"$cacheKey-v$cacheOffset")
        case (k, v)                => (k, v)
      })
    case x                                                               => x
  }
}

Start by tracing the githubWorkflowGeneratedCacheSteps setting and the generated actions/cache steps referenced in the issue. Determine how a configurable cache-key value should be exposed while preserving the default behavior; done means users can change cache keys to invalidate caches without changing dependency paths.