get_user_runtime_dir

Returns a directory that is unique to the current user on the local system.

This is determined using the mechanisms described in the XDG Base Directory Specification. This is the directory specified in the `XDG_RUNTIME_DIR` environment variable. In the case that this variable is not set, we return the value of get_user_cache_dir, after verifying that it exists.

The return value is cached and modifying it at runtime is not supported, as it’s not thread-safe to modify environment variables at runtime.

Returns: