The flake cache was only checking existence for paths starting with
NIX_STORE_DIR (defaulting to /nix/store), but not for paths in the
test store when CLAN_TEST_STORE is set. This caused the cache to
return stale references to paths that had been garbage collected.
This fix updates the is_cached method to also check for paths in
the test store, preventing cache misses during tests.
097024e8a9