To avoid the problem where multiple jobs try to create the same cache
entry, we add the GitHub job ID to the cache key.
The two restore keys make it possible that a job can also restore a
cache entry from another job if none for the current job exist yet.
With this commit we allow the Go version that is set up to be
configurable and not dependent on a specific environment variable. This
will allow us to eventually extract the action into a tooling
repository.
For some reason we used to override the GOCACHE and GOPATH variables
earlier. This now causes the updated cache action not to pick up any
caches. As the overrides shouldn't be needed anymore, we remove them.
