[fix] solidtorrents engine: store random bas_url in param

Two different threads ( = two different user queries) can call the request
function in a row and then the response function.  The namespace will be same
since this is the same engine.

To keep exactly the same value ``base_url`` must be stored in params and then
retrieve using ``resp.search_params["base_url"]``.

Suggested-by: @dalf https://github.com/searxng/searxng/pull/862#discussion_r799324861


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>