[SQLiteCache] Fix check if directory is writable (#5022)As reported in https://github.com/NixOS/nixpkgs/issues/467012, whentrying to use the SQLite cache on a read-only installation, rss-bridgealways errors out, because the directory it is in is not writable.This is checked even if the configured file path is an absolute pathpointing to a completely different, writable folder.This commit fixes the check to check the actual target directory.