Move modification time update of duckdb index file to libapi
EDIT:
After the explanation by @AndreaFrancis, this PR moves the updating of the modification time of the duckdb index file from the /search endpoint to the `libapi` function `get_index_file_location_and_download_if_missing`.
This way, the modification time is also updated by the /filter endpoint.

~~Remove unnecessary `Path.touch` in /search.~~

~~I think this is not needed.~~
CC: @AndreaFrancis 
Move modification time update of duckdb index file to libapi: EDIT:
After the explanation by @AndreaFrancis, this PR moves the updating of the modification time of the duckdb index file from the /search endpoint to the `libapi` function `get_index_file_location_and_download_if_missing`.
This way, the modification time is also updated by the /filter endpoint.

~~Remove unnecessary `Path.touch` in /search.~~

~~I think this is not needed.~~
CC: @AndreaFrancis 