Fix detect_changes to not override AUTO_CACHE_HAF_COMMIT in auto mode

In auto-skip mode (only tests/docs changed), we should NOT set
AUTO_CACHE_HAF_COMMIT from HAF_COMMIT because the HAF_COMMIT may not
have a cached datadir. Let smart_cache_lookup use the app's global
AUTO_CACHE_HAF_COMMIT variable (if set) to find an available cache.

This fixes balance_tracker failing to find cache for HAF commit
70c30036 when a working cache exists for 219ef447.