CAS daemon honors the runner’s TUIST_CACHE_ENDPOINT override

The Xcode CAS daemon now receives the runner’s TUIST_CACHE_ENDPOINT override via its launchd-agent environment. Previously the override only reached the binary module cache; without it the CAS round-tripped every compilation unit to the public endpoint, leaving the Xcode cache net-negative on a full compile on runners that can reach a private network cache. (Forwarding feature flags alone was insufficient because the server returns the public endpoint unless a hard override is present.)