Improve how the cache service availability is determined in actions/toolkit

[Link-]

For the purposes of GHES, we have a isFeatureAvailable() check that runs when attempting to use the cache service.

This function returns true if ACTIONS_CACHE_URL is set as an environment variable in the runner. ACTIONS_CACHE_URL points to the ArtifactCache service URL and is determined and passed to the runner by run-service or Actions Service.

In run-service, this value is defined in the Kubernetes manifests. In Actions Service, a call to MPS determines the value of this URL.

We need to modify the condition for isFeatureAvailable so that it's no longer only dependent on the value of ACTIONS_CACHE_URL but leverages the ACTIONS_CACHE_SERVICE_V2 feature flag as well.

Exit Criteria

1. Even if ACTIONS_CACHE_URL is not set isFeatureAvailable() would use another way to determine the availability of the cache service
2. Change the log level of failures from warning to error