Add `sticky-disk-key` input

[haines]

At the moment the sticky disk is always keyed by the repo name. That means if I have two jobs using this action running in parallel, they race to commit the sticky disk.

It would be helpful if the action supported a sticky-disk-key input that could default to the repo name but be overridden by the user if necessary.

I tried to knock up a pull request but was stymied by the private protobuf generated dependencies.